Hi, we went to Yala as part of our honeymoon and it was amazing.
Stayed at Chaaya Wild Yala which was wonderful.
I worked for many years as a wildlife guide in the Highlands of Scotland (Aigas Field Centre, if you want to check that I'm a real person,) and feel reasonably qualified to give some advice on the matter. (Though this is just my opinion based on my experience.)
Some of my personal feelings on guides:
A: I would not use the hotel safari service. I'm sure their guides are just as good as the local guides, but are outrageously expensive, and don't put as much back into the local economy.
B: The guides all seem good. The major issue I had beforehand was whether they were trustworthy or not. We booked with 'Sugathe' (which is the owner's name.) They were friendly and trustworthy. Also they did not ask for money until after we were back at our hotel. Don't be put off by the fact that their website is poor.
C: Leopards are the big attraction, and the guides will do all they can to find them. They visibly relax once a leopard has been spotted. (I know that feeling.)
D: .Unless you have kids with you, or have a bad back, don't bother with the 'luxury' option. The basic ones are comfortable enough, and the extra mobility you have is a real benefit especially for photos. (read on for anecdote.)
E: If you can afford it, go in more than once. (morn+eve or next morn.) It is a great place to be. If' you've already seen leopards, then you can concentrate on other things, but if you missed them first time, you can try again. I'd guess that almost all see leopards if they go in more than once.
We booked with 'Sugathe' at our hotel the evening before our morning safari. (Nothing like leaving it to the last minute to make you stressed.) He was so calm and matter-of-fact on the phone that once I'd hung-up I wondered if I'd actually booked it.
A young guide (called Shireen,) duly turned up in the morning and we went in.
Safaris in Yala are initially a little like the wacky races, but it all adds to the fun. While we were going around Shireen showed us many things from birds to elephants, and even a brief glimpse of a sloth bear. (That kid has eyes like a hawk. He spotted a pair of tree-swifts (small, fast birds,) driving at about 30mph.) When we finally saw leopards (male and female together,) more of Sugathe's jeeps were there (mobile communications helps to find things.) He got me to clamber across all the basic jeeps to the closest one so I could get in a better position to take some photos.
The cost was 10,000 rupees for two of us, which was so much cheaper than the hotel cost that we were able to afford to go out again in the evening. We didn't see any leopards in the evening, (missed one by 30 seconds according to the jeep that was there,) but saw some great stuff including elephants and black-necked stork. The boss' son came along to offer his apologies for this when he found out, but as far as I'm concerned, that's wildlife, and if you want guarantees, go to a zoo!
The hit rate with Sugathe for leopards is quite exceptional though, having read the reviews and given the time limit of around 3-4 hours. (Park rules for morning entry and evening exit, plus avoiding the heat of the day.) The number of people who see leopards is very high.
If I could go back to Yala, I would certainly go with Sugathe, and would definitely recommend them to a friend.
Hope this helps