I have a ProVari which I love and have had ego's which I have learned to hate because of the failure rates of
batteries. I will never again purchase a PV without a replaceable battery. I can get good quality batteries from DealExtreme for $2-4/each. shipped vs. $15-$25 for an Ego/Riva/Tornado style.
In all honesty, though, you cannot compare the ProVari with the ego. The ProVari is a top dog variable volt unit with many nifty but useful features, but there is a penalty in size and weight to pay for all of the goodies. It is also a lot more expensive. Then again, do you want a Ferrari or do you want a Miata?
The ego is a fixed but regulated voltage unit that has a whole system built around it, although you can use standard 510 threaded items (attys, carts, etc.). From the threads, the Jury is still out on the ego
tanks, super attys, etc. These accessories do have a price tag which is higher than similarly priced 510 items. To me, a regulated fixed voltage circuit is an OK feature but not a necessity when one considers the power output of a LiIon battery which drops off at the very beginning from ~4.1 v to ~3.7 v where it stays most of the time during the discharge cycle.
If you are looking for a smaller unit with a replaceable battery, then you might want to take a look at the Ikon from Pure Smoker or the ePower.
By the way, LiIon batteries do have a limited number of recharging cycles which is between 300-500 times. So if you charge your battery once a day, the ego (and all other LiIon based PV's)will give you about a year's use before it fails.
If you are not a heavy vaper and charge only once every few days, that battery life will be extended by quite a bit.