The newest entry level Mega Drive/Genesis Everdrive narrows the margin from the deluxe version. Unless you're hardcore into Genesis, there isn't much need to pick the deluxe version over the new basic one.
The Master Everdrive is great, but again only for the hardcore (just stick to the Mega Everdrive to play SMS games).
The Game Gear Everdrive is much more valuable, as no other hardware runs GG games and it has exclusives with fan translations (like Shining Force Gaiden III).
Everdrive for the genesis and snes are worth it.Though if you have the cash a mega-everdrive or the SD2SNES are fine.I don't have a opinion on the snes or nes powerpak though,when i heard about there flaws i avoided them.
All flashcarts have flaws. The Powerpaks had their major flaws fixed with updates.