I went in 2010 for the christmas markets (which were awesome) and before in about 2005.
Its great and I can thoroughly recommend it.
I did a special offer through BA and we stayed at Andels Berlin
http://www.vi-hotels.com/en/andels-berlin/ which is kind of a modern new age hotel thing.. really spacious rooms and good food although the sky bar was a tad pricey (it did have good views though). Great location about 10 minutes from town and right outside is an S Bahn stop (tram - there are loads in berlin and I think a day pass costs about 2 euro). Berlin is nice and very pleasantly hot in summer and very cold in winter.
As for Berlin itself there is loads to do, especially if you are into Post War/Cold War history.. the DDR museum, Checkpoint Charlie, Topography of Terror, Brandenberg Gate etc...
http://www.guardian.co.uk/travel/2011/aug/17/top-10-museums-berlin-city-guides. There are also tours and the Zoo if you like that kind of stuff.
There are loads of good restaurants and bars and the atmosphere is friendly and welcoming.
You must try a currywurst while you are there.. its like a hotdog with curry sauce and curly fries! Really good with a beer!
There are also nice places outside of the city like Potsdam which takes a while on the S Bahn but is a nice open market town.