In advance of getting a drum pad MIDI controller, I purchased this MIDI-capable drum machine. The DR-550 is part of the 'Dr. Rhythm' line made by Roland under the Boss imprint. I own two earlier models, the DR-220A and 220B, so was already familiar with how these machines function. They're quite fun once you get used to their eccentricities. A lot of functions rely on holding down the shift button, for instance, when dedicated buttons would have been more intuitive.
The original DR-550 was manufactured in 1986. I haven't been able to confirm when the mkII came out. The drum kit in the mkII was expanded to 91 sounds from about half that. Among these sounds are some fantastic 'big 80's drums' that I'm sure were a big selling point in 1986. It also contains 10 sounds sampled from the famous Roland TR-808, which is perhaps more of a selling point in 2013.
I've found that the internal timing clock is slightly faster than that in my DAW (it's a similar story with the Alesis SR-16). After much trial and error I was able to sync it via MIDI, but it's still not 100%. I found that letting the sequence run and using the second bar instead of the first seems to work best.
To hear what the DR-550 sounds like, I recommend checking out this page at Cyborg Studio* where you can preview the kit from the unexpanded machine, and also purchase the sample pack if you desire. *Please note I'm in no way affiliated with Cyborg Studio, but I think their site is pretty cool.
