I can't say I've ever had a problem using the Roland DR-660, although I haven't taken it that far in depth since I've only used the basic features of the unit. However, that is to say that the basic features and options are easy to find and use. There are too many different parameters to list, but upon playing around with the DR-660 for a little bit of time, I found I was able to have a pretty good grasp on how to use it for what I wanted to use it for, which was for making simple beats and rhythms. I've never seen the manual for the Roland DR-660, but I don't believe that one is necessary since it is easy enough to figure out how to use it without one.
While the sound of the Roland DR-660 might be a bit outdated, depending on the type of sound you're going for this can definitely work to your advantage. For me, I was just looking for something to get some quick ideas down, and the 660 worked perfectly for this. Since I was using it for this type of use, I can't really complain too much about the sound quality. However, I probably wouldn't recommend using this for a final product or song as with modern technology it is quite easy to get more realistic sounding sampled drum sounds...
Overall, the Roland DR-660 is a cool little drum machine that has held up over time. You can get these for a very reasonable price used if you are looking for one, but if you are looking to make some drum loops and beats and are new to the game, I would probably recommend going the computer route as it will give you a lot more options. This being said, if you take the Roland DR-660 for what it is I think you'll be quite pleased with it.