I use cloudbet. Don't really get involved with casino style gambling, but for sports betting. Couldn't argue with the 100% sign up bonus for being a new user (only been registered a couple of weeks and admittedly it will take time to get the bonus, but you get a year to earn the loyalty points and if I'm placing bets on a regular enough basis I should be able to reach the maximum bonus level), and the instant(ish) withdrawal option (after betting, when/if I win, I want the coins straight back in my wallet, or at least on their way back). The site definitely seems secure enough, I haven't read of any major issues anyway, and reviews seem positive. Also haven't needed to use the support as yet, but apparently it's 24 hours and they have live chat available on the site (will update if I have to use them at some point, which no doubt I invariably will).
I tend to bet mostly on football (soccer) and they have decent markets available - although not as in-depth as traditional sportsbooks, I am sure these will increase over time as the volume of action on games goes up. The high limits don't affect me, as only betting with smaller amounts on individual bets, and they seem plenty high enough anyway.
Just a couple of issues, one is the slightly limited betting options on soccer, but I think these will increase over time. And the second issue is the email requirement, but that was easy enough to sort, I just set up a separate email to use for the site. More than happy overall, but if anyone has any better sites to use for betting on soccer, please let me know.