I've actually had my waitlists convert a lot lately. Sideshow produces more than they did in the past, so it's been easier to have it convert. Just in the last few months I've had Thor Ex, Indy Ex, Storm Trooper Ex, and Darth Vader Ex convert for me. So I'd say it's worth taking advantage of.
The Terminator bust has been sold out for almost a year, so your chances aren't as good on that one. The waitlist almost always works if the item is still in preorder mode when you go on the list. As for this one, it could convert out of the blue one day.
Yes, I have had an item convert from the wait list. I highly recommend jumping on if there is something you want because their is a good chance you'll get it.
I've gotten quite a few. Sometimes I've been on the waitlist for months and it will convert. Never hurts to be on the waitlist. If you find it somewhere else first, you can always take yourself off.
Many collectors pre-order it seems just because, then cancel before there is any cost. Kinda makes me wish there was a nominal charge for cancelling and those of us who are serious don't have to sit around for months with crossed fingers. Done venting.
Most of the waitlists I've been on didn't convert for months (6-9 months) and I would eventually find them at online retailers or ebay. I've only had 2 ever convert (Belloq Ex and ANH Han Reg), and those were for items that I ordered fairly soon after their PPO.
I just had an E2 Obi Ex convert and I didn't get on the list until after he began to ship. Always give it a try and the worst thing that can happen is that you don't get converted. And in the meantime you can always be looking elsewhere for what you want and if you find it cancel the waitlist. If the waitlist gets converted you get an email notice and you are given 3 days to cancel the order if you found it elsewhere.