I held off buying this because of the issues with fit (with the new babyset). In fact, it's not a problem. We took the front bit off the baby set, leaving the back support bit and then fitted the play tray and strap. It fits fine and there's no way the baby can kick it off.
My almost-6-month old isn't sitting up unsupported yet (although he's pretty good with a little support) and he manages well with the play tray and strap. In fact it's a lot easier to get him into the high chair with the play tray than it was with the baby set - the plastic front of the baby set made it a bit of an ordeal since his legs don't seem to bend the right way.
The tray itself is great - a high rim to stop things ending up on the floor, easily cleaned and a great size.
The price made me wince but - as another reviewer said - once you get over paying £50+ for a bit of plastic, you really appreciate the tray.