There isn't anything difficult about using the DBX DB12, as everything on here is pretty self explanatory. Having said this, it does offer some features not normally found on a DI box, which is definitely something that I welcome. The box has switches for ground lift, polarity, a low pass filter, and a pad that can be set at either 0 db, 20 db or 40 db. A manual shouldn't be necessary, although I have not seen it so I can't speak to it's make up.
The sound of the DBX DB12 is quite transparent and it will definitely give you an accurate sound of what you send to it. It isn't going to color your sound, as it's strictly a messenger of information. It's nice that there are a number of parameters and features to use here, as it only extends the versatility of the DB12. I don't know that this sounds better than many of the other DI boxes that I've used out there that are cheaper, but I guess you're paying for the extra features with the DB12.
While the DBX DB12 is more than a solid direct input box, I'm not convinced that it would be my first choice if I were to need a new DI box tomorrow. While it's certainly sturdy and built well, the price is just a bit much for what you're getting here. Don't get me wrong - I love having the extra switches and features here, but without a real increase in sound quality when compared to other DI boxes like the Countryman DI, it's not worth the price for me. I'd still encourage those interested in a new DI box to check out the DBX DB12, but also check out those like the Countryman and others out there in the price range...