Yes, if you stop mining on Slush sometime during a round you do get hit pretty hard.
I've mined on BTCGuild before for about a week. NMC is a nice bonus, but for what I've mined there, even with the NMC factored in I still did a little better on Slush. So in my personal experience, I can get a better return mining with Slush.
I'm still mining a little bit with BTCGuild (about 10%) at least until I get enough btc to be able to get a payout.
I'm not going to get into a "my pool can kick your pool's @$$" arguement. I like BTCGuild, they have a nice website layout and the mining rewards are a little bit more consistant. If someone want to try them out, I'm not going to try and talk them out of it.
As a test I am mining on three different pools, with the same hash rate on every pool 24/7. Two with pps, one with DGM (Slush). So far I have a 10-15% higher payout from slush for the last month. It is quite evident where this comes from: pool luck in the last 30 days is above 100% and therefore I have received a 100% pay out of block fees and Transfer fees (which actually do add to the income!!!) On the other hand some pools calculate my income 25btc / difficulty - fee. So I consider this quite greedy and my loyalty will not be endless.
Obviously there is a psychological effect with the long blocks. If you are lucky and mine 10 blocks in an hour you are happy for an hour. If you are unlucky and gain nothing for ten hours you are unhappy for ten hours. I kind of have the Feeling that People get overwhelmed by there ffelings in a situation (mining bitcoins) where only logic counts. So if you are bored go and watch Television or talk to somebody, but dont stare at the stats or the block chain. This may kill your rational. Better look at your email accounts and also look at the 30 day pool luck.
Personally I kind of feel pitty with the pool operators as a whole. Transfer numbers increase, so should costs, but the income from blocks remains equal (at least in btc). As far as I understand Slush is constantly figuring out something, so lets wait and see.