Reliability with any of the s/c's are completely dependent on the owner:
- If you take care of the car (regular preventative maintenance) and run it appropriately (always 93 octane, only drive spirited when the oil temps ready to go), it'll be fine
- If you abuse the car & don't maintain it, your wallet's gonna take a beating
I'd contact VAC Motorsports about their experience with the VF Engineering VF620 kit. It's currently the E9x M3 stock internal 1/4 mile world record holder, uses the same Vortech V3Si Blower as the ESS (chosen for it's outstanding reliability), and they beat the HELL out of that car (regularly tracked on both the strip & road courses).
If ESS & G-Power are the only 2 options you have (TUV approval, etc), then it's a coin flip. The ESS can be purchased at a lower cost, but if you can budget it then I'd go with the G-Power just because I find it a superior, higher quality product than any other S/C on the market.
Also, EVOLVE's s/c might be another one to consider, just not sure if that has TUV approval (yet?).
Regardless, I'm jealous of everyone with an E9x M3 -- I'm dying to have that S65 power & exhaust note