From what I understand, 51%'ing a network does no harm unless the pool operator has bad intentions.
Why risk it though? What does ghash.io give you that BTCGuild or eligius doesnt?
Compared to BTCGuild: 0% fees
Compared to Eligius: less variance
I started mining with them around 3am UK time on 03.01.2014, so in about 1 hour that will be exactly 1 week mining.
My average hashrate is 3200GH/s.
For that 1 week period I have mined 8.54603089BTC exactly (not counting the last 1 hour of the full 1 week), while the calculator says 7.94166635 BTC per week.
So (8.54603089/7.94166635)*100 = 107.6% luck
So all looks good.