Got my Evo in December, like someone else said I'll be at the very least waiting 6 months before I upgrade to something that recently came out. Having said that, I don't see myself upgrading to the Evo 3D when I'm eligible but I guess it depends on what's available from Sprint and if the Evo 3D has a locked bootloader.
Having recently delved into rooting and flashing I can tell you I WILL not, if given the choice, buy a phone that is completely locked down. The Evo 4G is a beast but I was having lots of issues such as random restarts (GPS + Pandora at the same time for example), less then stellar lag in the UI, sluggish game performance. The straw that broke the camels back though was the gps issues I was having. CM7 quite literally has made my Evo 4g a completely different animal. That's not to say there are some issues (can't update PRL, clear GPS cache, and a few others without flashing back to sense and back to CM) but they are very minor issues that I can certainly live with. My Evo feels like a beast now.
So to add and recap, for me to ditch my Evo in 6months, phone would have to NOT be locked down, not be plagued with issues like the Palm Pre (phone gives me nightmares), BETTER battery life, and would have to take into account how my Evo is holding up. If it's still in pristine condition, I'm likely to forgo upgrading.