I've been thinking about switching to a single pin sight. Currently I use a 4 four pin sight that came with the bow. What are the good and bads about switching to a single pin sight? And whats a good single pin sight? I've been looking at HHA Optimizer Lite.

That's the one I have and I really like it a lot. I picked it up used off of ebay. I plan on only using it for 3-D and target shooting for right now though. I've read about several people that do use them hunting, they just set them for about 30 yards and then know what their hold is for closer or farther. Myself, I'd much rather stay with a fixed pin for hunting just for the ease of adjusting to an animal that moves closer or farther and still being able to hold a pin where I want to hit, especially elk when calling them in.
The sight picture on a single pin is really nice though, and when target or 3-D shooting I really like that I can dial it in to the exact yardage.
It's really a matter of personal preference as far as hunting, so that part will be your choice. I don't think you'll be disappointed by an HHA, it is a quality sight. They seem to have good customer service too, my brother in law had his level fall off and get lost. He emailed them what had happened and they sent him a new one right away.

Using the HHA Optimizer Lite for hunting. But I have hunted with one single pin for years. Love this sight. The light is awesome. The sight is very durable. This will be year three in the field with the sight. I follow the advice you noted. Set it at 25 yards and leave it there. I just make sure to practice slinging a handful of arrows at least 6 days a week. I just randomly move about the target, not measuring anything, as I practice. I move about from one spot to the next. From time to time, I also practice guessing distances by tossing an item (e.g. A ball) around and then measuring the distance with a measuring wheel. That seems to help with guessing.

Best of luck.

While I can see why some people like a single pin sight I prefer a multi pin one. For open country I would say sure go with it. But for me, I hunt mostly in heavy timber from tree stands. I range different objects all around my stand and should a "target" come into view they usually don't stick around long. They could be at 20, 30 or even 40 yards and I don't have time to adjust my sight to the correct range. And too, not saying that I would shoot at an animal that far, but I practice out to 70 yards.
The multi pin sights do not obstruct my vision enough to worry about it and if I have to shoot through a small opening I don't want to have to guesstimate my hold over/under. I can be more accurate with a multi pin sight.
I guess both sights have their pros and cons.