I rarely hate or like players TBH but I change my mind on their calibre all the time. It's inevitable - very, very few players stay the same all their careers.
Like you I thought Iain O'Brien was diabolical (he was) in his first 4 Tests and initially I presumed he was just another useless Kiwi domestic bowler who'd never be remotely good at Tests but I now think he was probably always a pretty decent bowler who just had a shocking time of it in his first few Tests.
Andrew Symonds is the most obvious example I always use - I thought he was an utterly hopeless OD batsman (he was) until THAT innings in WC2003. Obviously I didn't change my mind based on that single knock, but that knock was the start of the mind-changing and it was complete by the end of the WI tour of 2003 and Symonds is now undoubtedly one of the better ODI batsmen of recent times IMO.
Andrew Strauss I've changed my mind on twice now - obviously he started brilliantly and I like everyone had high hopes, but by the end of 2007 I was beginning to think he'd become a waste of space, wanted him dropped and wasn't amused when he was recalled immediately after he was axed. And I was gutted when he saved his place with that easy knock against a fourth-rate bowling attack at the last-gasp at Napier as I thought it'd mean we had to suffer his failures for another 4 or 5 games then. But instead the knock inspired him to completely reinvent himself and go back to playing as he had been originally and now I'm only too happy to have him at the top of the order once again.
Geraint Jones too - backed him to come good right to the very end of his Test career. Only after that forlorn pair did I really contemplate the fact he was just a very, very poor batsman who'd had his best season at the right time and would never have come remotely close to initial selection, never mind retention for 3 years, if he'd had that season at some other time.
I try not to come too hastily to many conclusions so I don't generally tend to jump in and decide a player is brilliant\crap after he's been on the scene 5 minutes but I was perhaps guilty of that with old O'Dire-to-O'Brill-en. Mohammad Asif was another. And I've changed my mind on him more times than you could wish for - first I thought he looked diabolical and a no-hoper, then like the best thing to hit the game since Curtley Ambrose, now currently I think he's a waster of the biggest proportion, and knowing the way of Pakistan I reckon I'll probably have to change my mind again before his time's out.
Oh, and on that note, even I was fooled once - back in 2000 or so - by the "has Shoaib come good this time?" I was wary of it on all subsequent 2456256 occasions, though, and I always turned-out to be right.