I use GUIMiner and mining.bitcoin.cz (Slush's pool). I don't think this pool has long polling.
I just switched to bitminter.com and try it for a week or so to see if there is a difference.
I am using GUIMiner at btcguild. Tried bitminter for about half hour to see how it goes. Personally it seems that the Java miner isn't as good as using poclbm in guiminer. I was getting about 380Mhash on poclbm with almost no impact on usability but on the Java miner only 355Mhash (despite upping settings and making the system rather sluggish) so you might want to continue using guiminer.