KarmaFX Synth Modular is damned excellent (probably one of the lowest-cost modular VSTi's available). While I don't have a powerful machine (therefore have to suffer with high CPU usage), the sound this thing can give out is outstanding.
Highly flexible routing options to obtain the sound you want, and it's easy to understand. The full version comes with a detailed manual with picture examples and clearly shows what this synth can do, in language most can understand.
The oscillator generators are clean, raw, and can be quite deep. With a maximum of 8 generators allowed of any following combination: 1-osc, 2-osc, sampler (not multisamples), additive, stereo pad and audio-input, it makes it quite easy to obtain a sound one wants.
It's got a great selection of filters, effects and modulation effects (a HFO, never seen that before in another synth! Never thought it was possible without breaking things.) The reverb and delay are pretty good quality. Chorus is a bit touchy to work with. I particularly like the chipper effect which switches around parts of a wave cycle to a different period based on its amplitude level. The Unison effect is GREAT. :D And damn those filters are obese! :)
My gripes with KarmaFX synth, other than the high CPU usage, is sometimes adding certain modulation modules to certain things will cause the computer to slow down considerably. Also, things like high polyphony and Unison really eat up CPU, and can cause a hang if you're not careful despite the highest limit hardcoded on the synth. Unison has a couple bugs with different, unconventional routings though that may be my fault. FM module doesn't sound the best and is a bit hard to use to get a reasonable sound out of it. And finally, the Pad generator's delay loading time. (in comparison to the similar-looking Additive generator.)
All in all, for about 150% the price of a full refill on petrol, this synth is a keeper. :D