I think she's a fascinating character. When she first showed up in the series, I was basically

- seen this type in a million stories. I felt the same right up until her chapters in ADWD. Before that, she seemed to me to be some combination of:
1. Actually control the events she says she can (Renly)
2. Retrofit events she had nothing to do with to make herself look better (Stannis losing at Blackwater)
3. Get lucky (Joff, Robb)
Pre-ADWD, I couldn't get a handle on how much of 2 & 3 were actually 1. Obviously, she had SOME powers (smoke baby, drinking poison) - Stannis DID lose at KL after she warned him he would if he didn't take her along and the three leech-kings DID die soon after saying their names (although, predicting a king's death in Westeros at this time isn't exactly going out on a limb). Also, Thoros can perform some unnatural acts in service of the Lord of Light too. So, her God apparently isn't a sham and she can sell the hell out of herself.
But her ADWD POV chapters show that she isn't the supremely confident character she portrays to everyone (even if she was really lucky). She's riddled with self-doubt and we see that she misinterprets her visions ("Arya" on a horse heading towards the wall - turns out to be the Kartstark girl). She also can't "see" Stannis any longer or what's happening in Winterfell with Mance (all she sees is snow).
I've seen some theories on what these misinterpretations may mean - "snow" = Jon which means he's the promised one (not Stannis) and she just doesn't realize she's had the wrong guy all along yet; that Arya may STILL come riding up that road in the future and Mel got the timing wrong - but I'm not smart enough to vet them out.
BTW - there's a nice bit of this "self-doubt" foreshadowing in an earlier episode of the TV show that we don't get in the books: when she discovers that Thoros can ressurect Beric, she is shocked that someone can do that.
So, she's certainly not as powerful as people think she is. But she DOES have something going on. I guess I still don't have a handle on how much of 2 & 3 are really 1, but the peeks inside her head have fleshed out what was once a 2D character. I'm really interested in where Martin's going with her story.