This is one of those cards that's not exactly required for all good Plasma decks, but can prove itself to be a godly asset in the decks that do use it. Stackable +20HP is great for allowing some of the non-EX Pokemon that want a chance to compete have that chance, or to allow bulkier EXes to have better walling options. Just a few of the cards that like this kind of support include: Beartic, Tornadus-Therian-EX, Absol, Weavile, Leafeon, and yes, even Deoxys-EX doesn't mind getting backup support from Umbreon (the two actually work well together, believe it or not).
Its attack is relatively okay too (DCC for 70, which can be powered up by DCE or Colress Machine and boosted by Dark Claw and Deoxys-EX), but unless Umbreon's the cornerstone of its own deck, your only reason for using it is for its Ability.
Ratings:
Modified - 9/10; very underrated; I expect it to start becoming popular as more people realize the value of those extra 20-80HP it can provide.
Limited - 8/10; you're not as likely to pull a whole wad of Plasma Pokemon to use with this, but it's still pretty useful regardless. And no Terrakion or Landorus-EX to get in your way, either.
Unlimited - 1/10; Plasma decks are gonna need WAY more support before they can be considered fit for Unlimited.