I've had several of these over the past few years - I am primarily a guitarist / vocalist but do play drums and keys on most of my demos.

The BlackKnight basically said it all (although I do not personally like the Alesis cymbals). There is a *tremendous* difference in the TD-30 brain modules vs any of the other Roland brains, or even any of the competitors. Alesis brains are basically drum machines with triggers added; they do not have the additional programming to catch the nuance of real playing. If you're a well-trained drummer, you will notice the difference.

There used to be a TD-20 which was very similar to the TD-30 in terms of the pads/cymbals, but the 30 is miles better in capability. It will catch your single stroke rolls and ghost notes correctly, and does not use samples for rolls, and your ghost notes can be extremely subtle. It's awesome, no question. Not quite like acoustic drums, but as close as you can get.

This tune may not be your cup of tea exactly, but it was recorded by a colleague, and the drummer did this song using V-Drums -

Isn't Superior Drummer what used to be called "drum kit from hell"? That is absolutely amazing when you're ready to record. IMO the stock Roland td-30 sounds are good for practice and demoing but the available samples from DFH / superior drummer are phenomenal in terms of flexibilty

It's kind of a pain to set up initially if you've never done this type of thing before, but it's fairly simple once you get the hang of it.

Depending on the software you use (I use a product called "Reaper" which is phenomenally supported, is extremely full-featured as long as you're not a DJ, and it really cheap), you can download profiles for each product doing a lot of the mapping for you - most 3rd party software has a guide out there. DFH/EZDrummer, etc is one of the best supported as far as I can tell.

To answer your other question - the drum kit becomes just a way to trigger these samples. The TD-30 actually CAN download samples to its brain so you can carry them around with only the brain, no laptop required - but I don't know all the specifics on this. When you're in a studio and you're running the kit through your processing software, the kit triggers whatever samples you have it set to. IMO it's overkill for most situations, and I'm a little suspicious of it for playing/recording drums 'live' - but it does work pretty well. Superior Drummer/EZDrummer/DFH sounds A-MAZING on previously sequenced drums.