I use the modcase. It's fine. The front reliably stays on, which is something some of those other designs seemed sketchy on.
It does block the top vent. It's fine for charging, but on rare occasion I (or the software) screw up and leave it running plugged in while the cover is on, and the overheating forces me to hard reset (I haven't hit the threshold to have it shut itself off, but the power management stays borked). For the price I'm happy enough with it, but I wish it vented properly with the front on.