Kings College Hall isn't too bad - it had a common room (with projector for films), a cool bar, Sky TV room, squash courts, table tennis, pool and full-size snooker table. The halls are split into three buildings, and in order of preference/niceness you want Platanes, Cameron, Danehurst. The actual atmosphere within the hall is awesome - because of the bar (and the mediocre food provided... apart from breakfast, which is quite nice) there is a massive social element not present at any of the other accommodation. You get to meet a lot more people, and mix with loads more people than just around your immediate room (which is the case at stamford and dover street, good luck meeting people outside your flat...).
The area itself isn't too bad. It's London. It *is* dangerous if you're not smart - but tell me a metropolitan area in the country that isn't anymore.
The are some great local places to go out - The Grove, The Sun and Doves, and believe it or not Brixton has some great night spots. A 24 hour sainsbury's is 2 minutes away from the halls, and is very handy for late night snack attacks.
When I was there there was no internet in rooms. There still isn't, but there is a computer room for everyone to use and there is wireless access in the common room for people to surf, it was installed 6 months after i left though. I had BT install a phoneline in my room, and i got myself 2mb bulldog broadband. I got myself a wireless router, and shared out my internet access to everyone in the corridor... i'd find people sitting outside my room at 3am as i walked to the bathroom in my boxers shorts.
Travelling into campus every day can be hassle. To get to london bridge/guys campus you have to get the train every day, which can take 15-20 minutes. Strand campus is reachable by train (train from denmark hill... walk from blackfriars - total travel time 25 mins) or on the bus, which quite frankly takes a stupid amount of time.
I arrived at KCH, and for the first few hours I thought "this is ****". As I got my computer and my hi-fi setup and bought some plants and posters and things my room looked brighter already. I went to dinner, didn't speak to anyone... went back to my room. Went over to the bar at 7pm, bought some drinks and from then I didn't look back. I have so many friends/mates now it's silly, and I really look back on my time at KCH fondly. I wouldn't have lived anywhere else, other than intercollegiate halls perhaps.
I hope that's helpful - KCH basically has some nice facilities, but it is quite far out from campus, and it isn't modern. But socially, it's ace.