WAIT - ask existing customers!
Having purchased both Things desktop and the iPhone app, I have to warn that Cultured Code is an almost hilariously dysfunctional company, with almost zero customer service, communication or integrity. They launched and started charging in January, long before the app was ready (it still isn't fully functional), largely because they had booked a booth at Macworld long before, assuming they'd be ready. They knew they weren't ready but went ahead and launched, not wanting to miss the Macworld opportunity. They promised that essential features would arrive with the first quarter of 2009 but nothing happened.
Worse, they made no effort to let paying customers know what was happening. This let to a storm of protests on their forums. Instead of facing up to the problem and posting on the forum, they decided to effectively hide the forums, removing all links to the forums from their main site. This deeply dishonest tactic led to ever more people paying for an unfinished product. Hiding the problems also helped them to secure several design awards, apparently based upon the glossy looks rather than actual functionality. I suppose it is too much to expect competition judges or reviewers to dig a little deeper into the problems that existing users are having, fair enough.
A representative of the company has recently started posting on the forum, making more or less the same promises he made in January. I am willing to give them the benefit of the doubt but, frankly, we have been here before - it will be interesting to see if they actually come through on their promises this time.
As it stands, however, I have to recommend that people avoid making the mistake that I made and invest time and money into an application that is unfinished and a company with no concept of customer service or communication.