TECHWRITER  I think the important thing to look at here is the magnitude of the upgrades. Version 5 is really bristling with new, compelling features. Curio is turning into a virtual 'control centre' for my day-to-day work. It's open all the time, both in the office and at home, so the value of the product for me is far, far beyond a measly $70 per year. If the next version adds as many major improvements as the last one, another $70 will be a bargain.  
(Version 5.1)

TECHWRITER  I can't agree that a good mind-mapping program is of limited interest, but I'd have to agree with TRYSOFTWARE that this program is very Windows-like. XMIND is a bit of a dog, and no way worth $300. Try out Novamind instead, which comes in three versions (the cheapest of which is only $50, and the most advanced of which runs rings around XMIND and is only $250 - and it's a SERIOUS business tool).  
(Version 2.1)