I've recently bought a boat from David Mawby at Castle Marina and am now mooring in the marina. I can only echo what has already been said, everyone is friendly and security is good.
The local run up and down the Nottingham and Beeston is good practice and Castle Lock is handy to practice locking which I have been doing since I got my boat. This week I've been up the Trent as far as Sawley and up the Soar as far as Kegworth, this should have taken little more than a day but I got stuck for 24h waiting for river levels to drop in Beeston and then had to wait overnight at Cranfleet waiting for the flood lock to be opened.
In short, in my admittedly short experience I would recommend Nottingham Castle Marina however if the River Level is up on the Trent you can be a bit trapped.