I have made a small profit out of
william hill and had no problem withdrawing (same with Casino and Sports). I mainly just use their Poker for their new user free-rolls and SNGs for a bit of fun, but I think they are relatively safe.
Regarding Paddy Power, I have only had 1 problem and it was at the Casino and I think it was my Internet connection. Regardless, I am still breaking even with Paddy, can withdraw about as quickly as WH, and just use them for the same reason - building microbankrolls. They give me free games bets every so often which I can convert into the Poker client.