I used to buy apps more often but now very rarely. It does seems I have all the useful apps I need already. That being said I don't often get new apps on my Mac either as I also already have the most useful ones and it isn't that often anything new gets written that interests me.
As far as iPhone apps go whenever I pay for one I always regret it these days. The last one I paid for was Shazam Red, as regular Shazam only allows 5 tags a month.
It really doesn't do anything for me that the free version used to. In the future I will be careful to upgrade apps if they are only limiting a feature to push their paid app. That being said I like Shazam so it wasn't any big deal.