I recently switched to OmniFocus as my GTD application for Mac OS X and iOS. Now, I realize this goes against my initial criteria for finding a new GTD system, which included a requirement that the system be cross-platform, but my computer usage — even at work — has shifted to almost entirely Mac OS X. Further, while I rely on an Android (Samsung Note 3) as my personal phone, there are a couple good applications — I currently use Quantus Tasks — that have been able to access the OmniFocus sync servers using an open API, meaning I can always access my todo list whether I’m on a computer or on the go. This has also made me more reliant on my company iPhone and I ended up purchasing the OmniFocus iOS application as well.
OmniFocus is not a light investment, the Mac OS X application is $40 (upgrade to Pro version is another $40) and the iOS app is $40 (upgrade to the Pro version is another $20), which is a turn-off for many users, but it can be configured from a very simple workspace to one with significant complexity and features, making it a great choice for GTD practitioners of all skill levels and needs. Personally, I’ve spent similar money on GTD applications ($50 for todoMatrix and another $20/year for its web interface, $20/year for Doit) and in my opinion, paid applications only increase the level and quality of support and enhancements from the developer over time.