I've been drumming for a few years and apart from the ION USB kit (which is awful) I don't have much experience of electric / electronic kits.
I wanted it so I could keep up my practice at home and also use it as a midi kit for music software. It fulfils those needs perfectly well.
I actually bought the DTX-50 model, it doesn't have any branding but it is sold under Rockburn and Technote in the UK, Medelli in the US and HD something in Europe. The DTX-50 and DD501 / 502 / 505 are all the same kit, made in the same factory, but just sold through different companies with different branding. (I was told this by a music store that stocks them and sells through ebay)
The frame itself isn't as sturdy as some of the higher end kits, but the pads are nice, good feel and a nice bit of rebound so you can get the doubles going much more easily than on an acoustic kit. I was especially impressed with the dual zone snare, which even has tuning lugs on it so you can alter the tension.
The Bad:
When they say compact, they realty mean it. I am 6ft 4, and it is too small for me both in height and width. The height issue is easily solved by placing some dvd boxes and books under the feet, but this does make it quite unstable, I will need to come up with a better long term solution. Frame is non- adjustable the snare arm and two tom arms can be moved to some degree but not much. The floor tom height cannot be adjusted as it on the frame. Not a huge problem though, it didn't take me long to get things in a comfortable position, and having the drums closer together will lead to more fluid, faster playing as I get used to the dimensions.
The bass drum attachment comes with spikes on it like some foot pedals do. I obviously didn't want to dig them into my wooden floor so I took them out. This means that the bass drum pad and pedal are liable to "travel" a bit. Again, not a huge problem if you have it pressed up against a wall, it also has Velcro on the underside so you can put a matt down and stick it.
The cymbal pads are of a decent quality, but the hi-hat pad is a little noisy, and the ride pad is dual zone which I was initially pleased with but it doesn't function that well. You can do a shoulder hit (as opposed to a tip hit) anywhere on the pad to get a bell accent sound, but it doesn't work that well, and I often end up getting a bell accent when I didn't mean to do it. This could be a case of getting used to it and learning exactly how to hit it.
The overall sensitivity and dynamics are obviously nowhere near as good as a real kit, and I'm sure higher-end kits will be much better, but it is good enough for practising and jamming.
The Good:
The inbuilt sounds are surprisingly good, the default setting is my favourite, there are a couple of different standard kits, and then you get into tablas, congas etc, and dancey DJ scratching sounds. These might be fun for someone who buys this kit as a toy, but I wont use them.
As I said the pads are also surprisingly good quality, and sturdy. I gave it quite a beating as I played along to some Kyuss, and everything held up very well, but obviously you don't want to go absolutely mental on it.
Where this kit will really earn it's pay for me is the USB midi function. I connected the USB to my pc (Windows 7) and it installed automatically, I like to use Ableton Live for music making / production and it received midi input from the kit. The only problem I've had is mapping each pad to the correct drum sound, but this is probably just a case of an afternoon's research and tinkering. It worked much better with Reason's Re-Drum program, with all but the crash and ride connecting properly. Very importantly I experienced no lag with either program, meaning when I hit a pad, the sound comes out of the PC at the same time, no 0.5 second delay. So lots of potential for creating custom kits and sounds here, for me, this alone makes the kit well worth the money.
Overall I have to say this kit is well worth the money, it's obviously not up there with the likes of Yamaha or Roland, but if you are looking to learn to play, keep up your practise or use it as a midi drum kit but don't have the cash for a higher-end kit, you wont regret buying this.