EV Mode Question - Extended use?

It kinda sounds a bit too good to be true! I'm looking at the one on hypridcars . com not sure who makes it.

Has anyone used it for an extended period of time and have it work? I have an 06 Prius and I'm a bit hesitant about putting this in. I have had it "drain" on my from sitting a couple times (when I go away for 2 weeks or more) and I've always gotten twitchy when the bars get low. Does this thing really work? A lot of my driving I would say is about in the under 34 mph range, but a lot of it is in the 50 and under range too. I used to do a LOT of highway driving but hardly any anymore.

Everything I read on here are folks that have "just installed it and it works great" or "not great" - I want to hear reviews from folks that have used it for an extended period of time - like months or years. Thanks!