We absolutely ADORE Kos.  We stayed at the Akti Club 2 years ago (2004) and are returning to the same hotel this May - May 27th-June 4th.  Can't wait!  The weather in August 04 was absolutely boiling hot, but there is a constant mild breeze which keeps it bearable.  I think it will be just about right in late May/Early June.  Kardamena is a fantastic resort, loads of brilliant bars and clubs, lovely beaches, nice people.  The Akti Club was, in our opinion, great.  Lovely pool, basic but clean and large rooms, right on its own (slightly shingly) beach with amazing snorkelling, food was quite nice and plentiful - better than we had in a much posher hotel in Crete last year - and go-karting just 3 mins walk up the road.  We loved it - so much so that we are returning and really, really looking forward to it!!