I have found 7 blocks in the just about 2 years I have been mining on slush. But ALL of those came in before ASICS, when my 1.5-2ghash was a decent number.
I haven't found any blocks in almost a year now...
EDIT: but I would like to say that my total payout during those 7 blocks was about 450btc. Which means I'm ahead of the game by almost 100btc. Definitely pays to mine with slush...
I have been mining with slush for about 5 months and I found 2 blocks, all be it I slowly up the hashrate with the pool. Wish I knew bitcoin back in 2011