Well, i've been using a J-station for a while now. You CAN get pretty good sounds out of it... But the factory presets suck. They're all WAY too bass-heavy and muddy as-is.
the easiest way to fix them (onboard EQ isn't enough) is to trade around some cabinets. I've noticed #5 and #7 (the two 1x12's) tend to be pretty manageable in terms of low-end response. Add a slight amount of reverb and maybe a touch of delay, and you're in buisness. This won't work with the amps that already have that cab, unfortunately, so you're forced to do some EQ tweaking after the fact, but it helps a lot. the two Mesa amps sound particularly nice through #7 (american tweed, i believe).
The other thing i've noticed is that it doesn;t do lightly distorted tones well- with an amp, it sounds better if you play slightly cleaner than you think you need to. the j-station's distrted patches sound better at higher gain levels- lower, you lose a LOT of that natural sustain and compression you get out of a tube amp's power section. Also, the clean tones don't have a lot of bite, even with the high's cranked; you can get interesting effects by bypassing the speaker and cab sims, and then double-tracking with a track through the speaker sim, or even just leaving it as-is for a satch-y sort of clean tone, but it's still not a really cutting, articulate clean tone. pretty mushy.
All in all, it works pretty well, though. Not as good as an amp, but a lot easier to work with at 4AM when you feel inspired.
If you want some MP3's, i can send you a few clips. send me a private message.