1. How does the card stand on its own? Is it good, bad, or have potential? Is it worth being played?
Espeon Prime has potential to be mixed in with any current other Eevee evolution, however outside of Umbreon not many Eevee-Evo decks are running around. It has 10 more HP than Umbreon, and can deal with the Fighting types that Umbreon is weak to. Should a Psychic type (or another Umbreon which will block it) come up however, Espeon can switch out to Umbreon.
2. What popular metagame decks could it be used in?
None that I know of, unless Umbreon becomes popular at rotation, in which case it could see some play. Should I run an Umbreon Deck, I'd run this too.
3. What combos can you use it with? How so?
See my previous two comments... In addition, I suppose it could be run with the Miasma Wind Leafeon. Both could use Miasma Wind due to Espeon's Pokébody. As the Miasma Wind Leafeon is commonly run alongside the Roserade with Energy Signal, Psychic (or at the least Rainbow) will be in there should Espeon need to use Solar Ray.
Another option would be to build a deck using all the Eevee Evos and Garchomp Lv X (Majestic Dawn) before it goes out. Depending on how the "pokémon you have in play that evolve from eevee" text is interpreted you could (in theory) put Eevee evos into the discard with Junk Arm and then have Garchomp Lv X bring them back as Basics. They are still technically Eevee evos, despite being Basic. If that is indeed the case, it could create some really interesting matches.
Running with the healing Serperior could also be interesting. Serperior heals 1 counter between turns, with Espeon Prime healing all pokémon for 1 using it's attack. That's a lot of healing. If spread ever comes back, this would be a good counter.
4. Give it a rating, and explain why you have given it that rating. You can also rate the card by rating the thread itself (out of 5 stars).
I'd give Espeon a 3.5 out of 5.. mainly because despite it being a fun card, it's yet to have anything good combos prove it to be a great competitive card.
5. How's the artwork?
5 out of 5. I think this image really brings out Espeons's mystery.