From my testing of the pool, I was having about double the number of stales as arsbitcoin.com.
Shouldn't matter; ABCPool pays for stales.
I noticed my invalid rate was pretty high to begin with (2-3%). But, I've had very few invalid shares lately, so my lifetime invalid rate has dropped to 0.78% over the past couple days.
However, I have also noticed that ABCPool stats generally report my hashrate to be between 1950 and 2150 MH/s, while my mining client consistently reports 2200 MH/s. One would think that if the Current Hashrate statistic on the site is a 10 or 15 min. average, I would be seeing my hashrate over 2200 sometimes and under other times. It's difficult to tell if this problem is affecting my income since I can't see any 24 hour stats to compare to my previous pools. I guess I'll have to start manually logging my balance at certain times of day.