Before Serum my main synth's of choice were IL Sytrus and NI Massive. Both of them had their limitations in my opinion. While Sytrus gave me the most amazing and flexible modulations i have seen so far, the limitations were its dull sound together with the fact that i can not morph a waveform on the go. Massive had a great full sound, and plenty of morphing ability's, but the modulations just didn't give me the all-round feeling i needed.
Serum changed all that, and then some. It combined the flexibility of Sytrus's Modulation, with the sound and wave possibility's i liked so much in Massive. Even though it offers just 2 OSC's instead of the 3 or 6 the others give, the sub OSC and Noise sections still give it a proper range of ability's. The lack of a second filter is partially made up for in the FX section.
The first thing i noticed was that it sounded really good. No matter what type of sound i try to make, it just sounds full and bright in general. It has a great selection of wave tables to choose from, as well as the largest number of Filter types i have yet seen on any synth. The number of possible FX are great too, and most of those sound excellent.
The real selling point for me are the LFO's. Even though they are called LFO, they lend the option to simply make them envelopes. The completely free multipoint options, as well as curve bending, speed, delay/fade and much more, make them among the most flexible i have seen beside the Sytrus. They can modulate just about every parameter on the Serum, including those in the FX section. The simple drag and drop make for a quick an easy workflow, and the clear modulation section gives complete control when needed.
When working with the synth i simply get the idea that X-Fer GETS what i want. The level of detail in Serum is amazing. "I wish i could snap to grid" means one look at the manual to see i can by holding CTRL. Making steps, or making steps snap to grid, it is all in there. This is one of the synth that make it really rewarding to read the manual, as there are a lot of hidden shortcut keys and futures. So far i have never had the feeling "I wish it had a function to do that" with Serum. Even better, the Serum user forum allows for contact with the programmer (Steve Duda) where you can request future options and exchange idea's. The life-time free updates makes sure you can enjoy those requests if Steve decides to implement them.
The wave table editor is also great. It's really straight forward, clear, and easy to create your own tables. Draw them in, or drag and drop. Really fun to play around with, and dropping 3rd party tables and waveform's (simply put them in a directory on your PC) is easy as it gets. Presets are managed greatly as well. It has a 16 voice Unison, and several different spread's. The controls over the starting point of the waveform, as well as the volume and detune controls make this one of the best Unison's i have ever heard. It has several "Warp" possibility's, including but by far not limited to FM, RM, AM, mirroring the waveform, warping the wavetable, and lots of other unique blending type's.
The reeverb isn't very good, but it rarely use the on-board reeverb in a synth anyway. What i do miss is a second filter on the front screen, and the option to take it serial or parallel to the first one. There is a second filter in the FX section to take care of any double filtering, but sometimes i just want to treat my noise to a different filter then the rest of my sounds. This is probably the only drawback i found in this synth that makes me grab back to Massive or Sytrus for some purposes.
It is a real CPU eater, though i have plenty so i rarely run into trouble with this. It is advised to keep Serum for those sounds that really need to be great.
TLDR version: Amazing sound, superb modulation, greatly detailed, very versatile, inspirational and unique in a lot of ways. If you have not tried the demo yet, go do so. It is worth your time.