I've personally purchased a small number of WordPress themes through ThemeForest and have encouraged a number of friends and clients to do the same for their own websites (most of which I have then gone on to install for them).
The service offered by ThemeForest themselves is pretty good. They clearly have a level of quality control that is applied to all of the content on the site so that you don't end up purchasing a theme which looks good but doesn't work. All of the themes I have purchased have always worked out of the box with the latest version of WordPress.
The only thing that lets ThemeForest down a little is that there have been a few occasions where the documentation for a theme has been poorly written and it's taken my a couple of hours to figure out why some features of the theme aren't functioning as they do the demo. The problem is that many of the themes use a variety of WordPress plugins. Some themes package these with the theme itself and others rely on you manually uploading or downloading yourself (which isn't always clear from the documentation). As such, you can sometimes spend a bit of time faffing and struggling to get it working as it should.
Most of the authors on the site do seem to take a lot of pride in their work and will fix and update their theme as/when it becomes incompatible with WordPress of a required plugin. Unfortunately, some others get left by the wayside a little and degrade a little over time. Never anything I've not been able to hack a fix too - but still, it's just a little bit of friction which prevents me from giving the service 5 stars.
Overall, I would still recommend ThemeForest to friends and clients, just make sure you take a little care to ensure it's written by a popular author.
Initially I was impressed with the selection and prices, but the real experience was much worse. You can't really get in touch with them and they take your money and then leave you alone with product creator, there is no guarantee that you ever get help from the author :(