I actually use an eGo too but, I just bought one of those Riva kits for my sister in-law. According to the guys that measure voltage on these things the Riva is actually supposed to deliver higher voltage more consistently than the eGo.
Below is a review from Scottbee. Looks like the Riva drops to 3.1 under load. Not saying that is bad. You really need to get into the big battery mods to have true 3.7 volts. An 18650 will have a max voltage of 4.2 but drops to 3.7-3.9 under load from my testing with a standard 510 atty. The eGo/Riva is still good in my eyes.
From Scottbee:
"I am not one to give reviews on products until I have had the opportunity to really put them through the ringer.... but all ow me to make some observations regarding the Riva:
1) The packaging is quite nice. Consistent with the type and style of packaging that we have come to expect from this class of product.
2) The batteries are quite attractive and nicely finished. The gold accents (in lieu of chrome) really turned out nice.
3) The button has a rubber/silicone texture and feel. Nicer than the hard plastic units found on similar devices. It has a good feel and a nice action. It is probably a little less prone to "accidental pocket firing" due to the lower profile and slightly smaller size. It is also probably less likely to get "stuck on" due to contamination and molding flash.
4) The LED underneath the button is blue. Looks good.
5) I was expecting these batteries to be "longer" due to the stated 750mAH. They are the same length as the 650mAH class units.
6) As noted in the advertisement, it comes with the "rapid" USB charger and a mains (115VAC) adapter.
7) The "effective" output voltage of the battery is 3.1V... same as the similar units (dern!), and all Joye 510 class batteries.
8) The atties that came with the unit are quite nice. I've never used an Eastmall 510 atty, so I didn't know what to expect. They are in the 2.3Ω range and have an excellent supplemental wick. They are of the "wide band" variety. No internal washer.
More later I'm sure...."