I used a 5,1 Mac Pro (not fully specced out) for about three months, just to see how long I could use it for. I used it almost exclusively (so I hardly used my M1 at all during this period). It was really slow, especially compared to Apple Silicon. The CPU I used was part of the problem, because it was one I pulled out of a server board, so it wasn't very fast—I'm sure the X5690s are far faster than the one I used, but it's still not even close to Apple Silicon.
I wouldn't recommend getting one, because you'll not only have to buy the machine itself, but you'll also be spending a good amount of money on upgrading a 13-year-old computer. At that point, it just makes sense to get an M2 Mac mini or something, which is way faster and consumes far less power. Unless there's a very specific reason to have a 5,1, you should avoid it.