I bought a pretty complex inventory system this week. It allows for all kinds of neat stuff, from banks, looting, crafting, equipment management, stat management, etc, etc. So far I am quite happy with it. The documentation could be better. It's taking a little bit of time looking through the code to figure out specifically how some stuff works, but overall it'll save a ton of time.
Also tweaking the character controller a bit so it's more responsive. I was getting pissed off trying to jump on the rock formations and constantly taking an extra step and falling off. Basic movement controls are a lot tighter now.