I did a fair bit whilst a school kid with the public school in Staffs I was imprisoned within. Its great fun but very tiring on the upper body. But I loved it, I'd do it now if I could. Great fun. Especially on a nice summer day, floating down a river.
For a superb introduction try these people Sea Kayaking Holidays in the Hebrides & West Coast of Scotland who are based in Barra.
I've got a Prijon Seayak which is a stable platform but the issue for me is the lack of knee room which compromises the overall comfort.The longer the waterline length,the faster the boat generally but with slower handling and in strong crosswinds,a rudder may be needed to maintain straight line progress.Its great fun but allways be careful in open water.