Also came over from BB/UberSocial so I know what you're looking for/expecting.
IMO, the native Twitter app (just called "Twitter") is absolutely fine. It is free. Certainly loads tweets and photos much more easily than BlackBerry ever did. It allows out to retweet or "quote tweet" where you can add your comment to the quote, which I recall a lot of BB twitter apps did not allow.
Then there's TweetBot, which is $2.99 (but apparently goes on sale from time to time). That's the paid Twitter client that I've heard most people swear by. It adds some presentation and some neat sound effects...i can't speak to it because I haven't used it, but perhaps it adds some other features for convenience.