Lots of people hate on old Tascam's, but the 520 is still a fun beast. The main thing is to not compare it to modern 'clean' mixers, nor to vintage transformer-based mixers. It's very much a product of the 1980s; if you like that sound, you'll probably like the 520.
Pros:
- Good ergonomics and feel (pots, switches), and reasonably well built
- Very flexible routing, with good workflow for 8- or 16-track recording (what it was designed for).
- Excellent manual.
- Nice big meter bridge with all eight groups and stereo busses.
- Pres and EQs are useable.
Cons:
- Almost everything connects via unbalanced, -10db RCA connectors. I don't mind the RCA connectors (they're fine as long as you aren't plugging/unplugging frequently), but I'm slowly realizing that the lower signal level and the unbalanced connections are a hassle when most of your outboard is +4/balanced. You'll be wanting to keep your racks pretty close to the mixer -- no 20-foot snake runs!
- Every input *must* go through both the input strips and one of the 8 groups, and then into the stereo mix. This isn't the quietest mixer, so adding another amplification stage is not optimal. (It's probably possible to short-circuit this by using group outputs, but it would make things more complex.)
I bought mine for $350 (I think) and then had to put in a few hours of time de-oxidizing and lubricating all the pots and switches. That was a good lesson in how the thing works, but if I did it again, I'd try to buy one in better condition.
Hope this helps.