Bet365 is a huge UK company (one of the biggest private companies in the UK) and is one of the most famous bookmakers around. Therefore you can generally be assured of several things.
A) Fast payouts
B) Descent support
C) They won't much you around with the bonuses!

This was the first sportsbook I ever used online and I was as impressed then as I am now. Not only do they offer great monthly bonuses they have a great initial deposit bonus. Bigger players that deposit £x,xxx a month may be entitled to extra privileges such as higher matched bonuses and loyalty rewards.
The initial deposit bonus is a 100% match up to £$100, you can then place 4 bets up to £$25 and have them matched straight away by bet365, once you have had your bonus matched you must then place another bet using this bonus credit.

Bet365 offer a massive variety of sports to bet on, as well as the obvious football and horse racing they offer betting on slightly more obscure sports as well as specials such as 'dancing on ice winners' and 'pop idol' winners so if you think you know your TV get your bet on quick!! Betting in play is also an option for certain football games as well as most european matches.

Betting is very easy to use through their web based interface (no download required) the betting system is easy to use (you can choose between decimal and fraction odds)

Banking is also very easy with the usual array of options such as credit/debit card, neteller, firepay, bank transfer etc. Withdrawls generally need to be made by the same method as which you deposited. (however you can change your deposit method at any time)

Bet365 is well worth a look if you are a big sport betting fan or just want to claim the £100 signup bonus. Constant punters can be assured of fast and reliable payouts as well as good customer service and loayalty rewards.