Our DS is only 9.5 mo, so we don't have a lot of pressure to switch to sippys, yet. But DS is extremely independent (pulling up since 7 mo!), and has picked up on the sippys really well. What's working best for us right now is the old-fashioned cups that just have a spout with a slit. Our only problem is that they only hold 7oz.
We started with the Nuby soft spout with water when we started him on solids at 4 mos. LOVED water. (It helped that we had a really hot summer, so he was probably thirsty a lot.) Before we offered him any other sippys, I'd test them myself. If I couldn't get a drink out of it, we didn't offer it to him. (That cut out all the ones with no-spill valves.) We stuck with the Nuby until a month or so ago, and started experimenting with some hand-me-down cups from a friend. Most of those were just too hard to hold or drink out of.
For the past couple months, he's had an open, double-handled cup in his bath - to play with. He's actually pretty good drinking his bathwater with that.

(We save soap til the very end of bath.) Tried the avent sippy spout (minus the valve), but he didn't really go for it. We had a freebie sippy - the type with just the spout/slit - that he showed an interest in. That's when I bought more with the same type lid. Now that's the only type he'll take.
He also has a game he likes to play, that might've helped. He takes a big, plastic cup (one of ours - like 32oz maybe?) and puts a tennis ball in it. Then he acts like he's drinking out of the cup, and the ball bumps his nose, then he tips the cup down again. Over and over. I just showed him how to put the ball in the cup... he invented the game.
Anyway (sorry, I get wordy when I'm up late), we now finish some meals with formula in his sippy cup. He seems to do fine with it. I think he's just young enough that - at least for some feedings - he still needs the snuggle and ease of being held and drinking from a bottle.
Hope something in this helps...