It's not consistent at all, but IMO that's part of what is interesting. This AM I've picked up quite a few TX fees, but for the most part it is mining for 0 reward. The reward is based solely on the amount of transactions in a block. The fuller the block, the higher the reward.
Edit: there are also people willingly adding fees to TX - so when i say "The reward is based solely on the amount of transactions in a block." I guess that isn't quite accurate.
Sounds good, i've pointed my solitary R290X in support of this to try and hoover up some small fees, and have added PMC to my sig.
I'll run a give away this evening when I get back from work.