A revolutionary program it is indeed since I used version 4.

Since the early days of ACID, it revolutionised the use of WAV/OGG/other files with the ability to paste and loop into a project, which can be altered in pitch without changing speed, and it'd still sound relatively fine. As the versions progressed, now there's even better support for MIDI/VSTi interfaces, multitrack recording (though I don't use that), and most importantly flexibility with Audio Stream behaviour on the timeline.

I mainly use this program over anything else as, once you put in a single sound file into it, you could do so much there's no need to save an extra file for let's say the same sound at a different pitch or speed, amp, pan, volume, or whether it's reversed gapped et cetera. ACID is capable of doing all that straight on, on-the-fly, and performance isn't any problem. Of course upon major chopping and enveloping, it does make the timeline a lot messier than it should look.

The GUI is left unchanged for the most part, but a bit of exploring (zoom in with mouse, enlarge track lines) you are presented with even more options under the hood. Even the groove tool is very handy. A bit of keyboard shortcuts, you'll find mixer tracks are automatable, it's practically got all the control on your fingertips.
And with the major support increase of MIDI files, now they all can be imported effortlessly. IT's possible to cut/paste events from there to a different place which is quite handy.

There are however some major downsides to this program. One, performance. This program does not go well with older computers and tends to stutter/lag when there's a lot going on, or in general, as ACID takes up a lot of RAM upfront. Two, the VST plugin handling system. It scans everything and sometimes can crash if a certain plugin doesn't work. Some other plugins such as vocoder plugins (except g200kg vOcOv, an awesome free Vocoder VSTi) or pitch-tuning plugins do not work at all. Three, Tempo changing. It still uses the marker system for tempo changes. Unless I use ACID as a ReWire client to something, using ACID alone for tempo changes is quite painful. It can read MIDI tempo messages but it doesn't work on the project itself which sucks really. Four, stability issues. Sometimes this program crashes randomly. When something doesn't work, it crashes the whole thing. Lastly, it takes so long to load the plugin list.

All in all, a very very handy DAW, though it still could use a bit more adjustment on tempo support, stability, plugin list handling, and speed. (Compared to any other DAW, ACID is a stutter-prone hog) It may be designed for loop-users, however it has the potential of being used as a proper soundfile sequencer, albeit the best one available out there. (Edit all sounds inserted straight onto the timeline!)

Isn't it odd? Only one review, and that of an expired version. Let me try to redress that problem.

I have taken a very odd path into recording and composition on a PC. I started out with Power Tracks Pro
(now in version 10) and Band In A Box (a sequencer based on families of musical styles from PG Software, which can output to MIDI or WAV). The results were/are muzakish or karaoke, and a lot of tweaking is needed to get any funk out of them.

Which is where I discovered ACID, looking for, e.g. ways to reverse guitar parts, or to handily play back drum WAV loops (I tend to write from the bottom up, and MIDI drums drove me nuts!). ACID isn't the perfect medium by any means, but it will do a helluva lot more than people give it credit for...

GUI: Very clean and clear, largely unchanged from Sonic Foundry days. You get an 'infinite' number of tracks into which you can put WAV or MIDI: either loops, one-shots, pre-recorded, or live. There plenty of processing (Fx, Volume, Pan, etc) at the track level, as well as on assignable buses for mixes.

One major improvement in v.5 Pro is that you can cluster parts in a folder (very handy for saving screen footprint when building complex rhythm parts). The Pro version comes with the XPress versions of Native Instruments' Hammond B4, FM7 and P53 ($117 worth, free). It'll host pretty much any VST I have tried, and multiple instances also work. I have a huge family of VSTs (free and purchased) and it's never complained or failed once. ACID is also rigged for ReWire and hosts DXi plug-ins, too.

There are plenty of Sony's own Fx built in, but nothing stops you adding more in VST plug-in form.

What can't you do? A common beef is that you can't assign envelopes to MIDI tracks in the same way you can to WAVs. This is a feature I hope will be added in the next full or partial upgrade.

The program takes a while to load -- all those VSTs, FX and my vast (100GB+) loop library being a cause, I'm sure.

Whether it's a great studio product or not, I can't really tell you. It works for me: I tend to use a lot of loops as shortcuts, and add MIDI or real audio at a later stage (or, when I find myself saying: 'this is where the M-Tron plug-in fits in,' or 'there's a better pad in Crystal,' or 'I'm going to work exclusively with Wusikstation today')

I also make my own loops at times -- you can do it from ACID Pro, but I prefer to use Sound Forge.

I'm conscious sometimes that I'm building my workflow around some of its quirks, but ACID seems like a good 'songwriter' product, capable of commercial-quality demos, and even commercial product. But I'm quirky, too. Sometimes a song will start out in the 'real' world, on my Tascam hard disk recorder with click track, guitar(s), bass, drum machine, vocals, migrate briefly into PTP for Helicon twiddling of vocals, then pop up in Sony as a package of WAV tracks. Then, loops get matched to it, and MIDI appears, via VSTs. Other times, the ingredients will be mixed another way.

So, ACID Pro works for me right now, and probably will for a while. It'll either grow to rival products like Cakewalk, or set itself some more modest boundaries. Only Sony can tell you about that, and they don't talk.
It's conceivable that I might adopt some other product in the future, but I haven't got a clue what it might be: the alternatives all look rather over-priced
and equally idiosyncratic, to be honest.

BTW, the program only costs $299, direct from Sony. And you can convince yourelf whether you want it from the 'home' version, which is around $70.

Edit: In April '06, Sony updated ACID to v.6. An upgrade from one 'Pro' version to the next was $130, for a short time. I've only used the demo of ACID Pro 6, but it seems to resolve a lot of the reservations expressed about handling of VSTis, and of MIDI tracks. While it still lacks one or two features of regular big-name hosts, it can outperform them in many other ways, if you're a loop user. My workflow is now increasingly 'all ACID' or 'all FL Studio Producer Edition,' depending on the type of project.

What started the loop based revolution has seemed to hits it's peak.

I like it's traditional track-base workflow. You can drag in loop files into a track from a CD or hard-drive with it's integrated file browser. Effects are added on each track instead of on a master mixer (which I personally like) and there's an integrated window for tweaking those parameters.

I've picked up a few Acid CD's along the way and they are great or making quick tracks. Of course you really need a large number of disks to really get the most out of Acid. Or... do what Ido and generate loop files from FL Studio and bring those in. That is a great workflow as you'll often want a different sound than what sample loop will give you.
So in that sense Acid makes a great audio sequencer.

I've tried using the MIDI in v4 and it's such a pain it's not really even worth fooling with. I've even hooked up FL Studio to it via ReWire and recording audio stright into Acid from FL studio but that's a bit of a chore as well. For me tweaking soundsand soft-synths in FL Studio and adding as an audio file into Acid works best.

I'm not sure if Acid is worth it's current market price. It depends if you want to do video scoring & surround sound. I picked up my version at a used bookstore in a discount bin for an unmentionable low price. The tool is best left really for those with large sample banks who get paid dollars for doing commercial work and need to do it quickly.