The biggest limitations of the 840 are:
1) Compression. You can't escape it, and I hear artifacts on my tunes, and the artifacts become more and more noticable with bounces.
2) The two linked Stereo channels. This is NOT an 8 track deck - it's a six track, and there's nothing you can do - short of using an external mixer - to change that.
3) You'll only have four direct outs - Left Main, Right Main, Aux A, and Aux B - but this won't matter until you get the external mixer to try and solve the linked stereo pair problem.
4) It's extremely difficult to rely on just the internal effects. They're basically global effects, meaning that the effect you select is the ONLY effect availabe to all tracks. Yes, there are multi-effect patches, but even still you're stuck with those for all tracks. So, if the multi-effect is a small room reverb and a single tap, eighth note delay, you can't dial in a plate reverb with a double tap quarter note delay for the vocal. So, sooner or later, you'll be thinking about using external effects.
5) But, you can only return external effects to open tracks or V tracks - or use the external inputs on mix down - but they're panned hard left and hard right, and I've never found a way to change that - so you can't place a mono effect anywhere but full left or full right...
But with all of that said - this is still an amazing machine, and you can make some good recordings with it. You can have a bunch of fun with it, and you can learn a lot from it. At current prices, it's a screaming steal.
Before you do put down your cash, though, ask yourself what you really want to do with this machine, and what you want to get out of it.
If you're interested in making demos, and you're basically going to do no more than record yourself and another person, and you're using MIDI tracks or drum machines or loops, this is a great tool.
If you're thinking you're going to be recording masters with a full band - you may want to look at a machine with more tracks, no compression, and better (more versatile) I/O.
After two and a half years, I'm moving away from a VS based system. But, I'll still keep my 840 because it's portable, useable, and a great tool for working out and capturing ideas.
I've had a blast with mine, and don't regret buying it in the least.
From your other question about
the TM D 1000, I think you'd be frustrated with an 840. You might want to look at an 880 VSR - the rack version of the 880.