The configuration of the 166XL is straightforward and is your basic compressor. It is easy to get a good sound out of it and is capable of getting a couple of different types of sounds out of it - ranging from a light compression sound to really squishing the signal, and it is also a sufficient gate. It all depends on what you'd like to do with it. The manual is pretty complete and was helpful when setting it up with my configuration.
The sound quality of the dbx 166XL is a little bit above average in my opinion. While its pretty clean, the sound won't blow you away. It is a basic compressor and is definitley helpful to have a few of them around for recording drums if you like to track with compression. I have used this on snare drum, kick drum, guitars, and bass guitar. I like it a lot on drums, as it will fatten up your signal a bit and is clean. Its faults I think come in the that it sometimes gets too tinny, but if you use it in the right situations, it usually is reliable and sounds good.
I've been using the 166XL for a few years. They are a real cheap option for the home studio owner who wants a few compressors to use for tracking. They sound good and are easy to use, while being quite well built and reliable in terms of it not breaking down. I like this model better than the dbx 266XL, but they are pretty similar in nature, I just think that the 166xl sounds better and cleaner, and is probably worth the difference in price. I definitley recommend those home studio owners on a budget to check out the 166XL as a reliable option for using compression.