So what exactly is so great about the Pila 4 stage charger? The tenergy that I bought with the ProVari seems to work perfectly.
Personally I use an i4, which isn't a full 4-stage charger but it does offer some of the same features.
Pila, according to the battery/flashlight nerds, has been one of the best chargers on the market for quite a while. The idea is that it checks to see where your battery is at (1), provides a fast, 'aggressive' charge to get the battery close to full, quickly (2), slows down with a 'softer' charge to top it off (3), & then goes into a trickle charge mode (4). Charging can be pretty tough on batteries, especially if you're providing them with a full charging current when they're almost topped up. Trickle chargers are nice because it doesn't just shut off when it hits full. It will maintain that full charge. Not that you should leave a battery on a charger longer than necessary - you're gambling with the chargers electronics at that point. Sure it might be ok most of the time, but that one time when something fails... not something I'm willing to risk personally.
I looked into them, but from reading on the flashlight forums, a number of those guys consider the gen2 Nitecores to be almost in that same ballpark... at half the price. I recall a few of them noting that Pila's can be pretty noisy as well, which I didn't care too much for.