I am a previous owner of Manytone's Ultimate Bass Kit sample set, which has been my go-to bass sample set (in Wusikstation) for the last few years. I wondered if Manybass would be sufficiently better-sounding to make it worth the $120 purchase (four times the price of UBK, which is admittedly a tremendous bargain). Short answer: it is, and then some.
Manybass consists of a sample set and a playback+effects engine (based on the sampler engine from AraldFX) to play them. The samples are compatible with Wusikstation as well. The bass guitar sounds have a realism that's very exciting, and they are supplied in a range of articulations (picked, fingered, slapped, etc.) that can be layered into a multi-sampled keyswitched patch for performance. The range of sounds covers just about anything you would want a bass guitar to do in a song, and the basic tones are augmented with a very useful set of effects that includes amp simulation, cabinet simulation, compression, EQ, phasing, and so on. The value of these effects is increased by an "FX Wizard" that basically asks what you want the bass to sound like (punchy, soft, deep, etc., etc.) and then sets everything up accordingly. Fast, effective, and a good learning tool too--I wish more of my synths had this feature! The GUI overall is beautiful, easy to understand and use, and a pleasure to work with. Several dozen presets are supplied, with names that make it easy to understand at first sight what kinds of material they're good for.
The amp and cabinet modeling aren't very extensive--you get only three amp models, for example, though a much wider range of cabinet models--but what's here sounds very good and adds plenty of value to the package. The cabinet models in particular produce a wide range of tones that can fulfill any number of roles in a mix, and are well worth exploring for any given song. I happen to own the Line6 Gearbox Gold plugin (which itself sells for a price higher than Manybass), and of course the range of amp models, cabinets, and effects in that package far surpasses Manybass's; but if you don't own another amp modeling plugin, the one supplied with Manybass will work fine for a wide range of musical applications.
The application has been very stable for me under Sonar 6.2.1 and uses relatively little CPU. It takes a little while to load a sample set on my Athlon x2 4800+ machine, but that's par for the course with big sample sets. It's a lot faster than Wusikstation in this regard.
Manytone supplies additional soundsets on their website for free to registered owners, and the plugin will play Wusikstation soundsets in addition to the bass sounds supplied with Manybass. I'm looking forward to hearing what Manybass's effects section will do with some of my Wusikstation electric piano soundsets in particular. I have had occasion once or twice to email the developers, and they have replied promptly with useful information.
The volume level of the presets is a little low under Sonar, something that Computer Music noted in their review of the plugin also (for whatever DAW they were using to test). It's not a big problem, but I'd like it if the plugin put out a little more level.
That minor issue aside, this plugin offers excellent value for money. There are certainly a wide range of plugins available, at multiple price points, that produce great bass sounds. For that matter, free plugins are avialable that do a good job on a range of synth-y basses. But if you're looking for something that does bass guitar in particular superbly well, this plugin is well worth the moderate asking price. I can't imagine that you'd need anything else to do great bass guitar lines, except maybe a great bass guitar and a killer bass guitarist to go with it.
Update on May 21 2008: Manytone is having a sale on their Ovation Acoustic and Upright bass sample sets, and I decided to lay out the additional $39. These sets are of excellent quality, and add signficant range on the acoustic-sounding side to Manybass. The presets that come with this package, however, are not designed for Manybass--either Manytone's Manyguitar or Wusikstation is required. I own Wusikstation, and the presets sound great in that synth. (In fact, they show exactly how great Wusikstation can sound when the sample sets are top quality.) If acoustic bass sounds are a big part of your music, and you own either Wusikstation or Manyguitar already, this package by itself is an excellent purchase.