Ok, so I have been to Safaris in Africa, so this obviously pales in comparison. However, even to Sri Lanka standards, it was not that great and overpriced, as usual. When we went the weather wasn't bad, but the weather the previous days was very bad, so apparently many animals weren't going out. The ride and landscapes were nice, but we were not very lucky in animal spottings. Other than the fact that we only got to see one lone elephant, we did not see any leopards - which can't be guaranteed anyways. We saw a lot of the usual bores and water buffaloes, lots of nice birds (especially peacocks), a few common deers (not the barking ones) and a couple of alligators. The coolest things we saw were the muskrat, which I had never seen, and a huge porcupine, which is apparently pretty rare to see there.
I would still recommend going on a safari in Sri Lanka, if you get the chance, though I'm not sure if Yala would be the best place. Only go there if you want to have the chance to see a leopard, no guarantees - which is something we knew. I heard that Wilpattu may be better, because it is less visited then the high traffic Yala. If you have the chance to visit several times (pricey), then you would probably see some. In hindsight, we probably would have enjoyed the nearby Udawalawe more, because you are virtually guaranteed to see large herds of wild elephants there and I heard the landscapes are more beautiful there, as well, though Yala's were also pretty nice.
I don't blame our driver, he drove us at least a couple extra hours, for free, searching for leopards, but no luck.