Here's some reviews:
The last one I'm a member of and it's great when you've made a song and need some feedback, the guys there are the greatest!
The thing about Reason is that it replaces a whole studio. Hook up a modern MIDI keyboard/controller and you can tweak the knobs of the built in (or plugged in) virtual synthesizers. The wave output of the program is mixed, reverbed, distorted etc with very high precision - all the instruments give perfect wave outputs and the resulting wave is "calculated".
Apart from the effect rack units it also has mastering capabilities like compression, EQ, tape warming, etc. It's supposed to replace a whole rack of rack units in a studio. But it's not supposed to replace the analog/acoustic part of a studio, to bring acoustic stuff into Reason you ReWire it to some kind of sample recording/editing software.
The reason I like it is because it doesn't feel like you're using a piece of software, you turn knobs and listen and plug cables into rack unit to build a rack for your piece of music, remix, add some compression to the drums, etc etc. It has a much more analog feel to it.
To make fairly advanced compositions, like 20 instruments at once, I guess you need a 1600+ AMD CPU or better, because it's the CPU that "calculates" the music. The rest of the PC (or Mac) can be pretty crappy because (afaik) it only uses two audio channels and isn't really heavy on the graphics.
OK, didn't mean to hijack the thread, just wanted to mention the MIDI software that made me stop using B'n'P, now back to the topic! :P