I'm going through the ER right now (I know I'm late to the party), and a own a copy of the game that I haven't really dug into. Is the story and game compelling enough that I can watch the ER, have the story spoiled for me, and still find a lot to enjoy about the game, or is this really an either-or thing?

I watched the entire endurance run before playing through and still enjoyed it. I also watched it again while playing so I could get the funny commentary in my game... or at least I pretended. Try it out, if you enjoy the gameplay then yes it is still enjoyable. I still got some kicks from the story, even though I knew what would happen. I still enjoyed watching it unfold for myself and at my own pace.

I definitely recommend that you play through the game first. Experience it for yourself first and then watch Vinny's and Jeff's reactions to the scenes.

Play Persona 3 instead then move on to some other Shin Megami Tensei game so that you actually play something other than Persona 4.

I played it afterward, really enjoyed it. In fact I'm playing it again right now

If you have it I say play it first. That's the reason why I haven't managed to finish watching that ER because I got a copy of the game and want to finish it for myself first which I haven't done either.

Play it first, then watch the ER for the good times and for Jeff and Vinny's reactions

I watched the ER up until episode 118 (Just checked), played through the entire game, watched the rest of the ER, and enjoyed them both immensely.
So yeah, I would suggest to stop watching, go finish the game yourself, and then come back.

I never watched more than I was currently up to in my own game, I think it's probably better to experience it first.

Stop watching once they get past where you are.

Playthrough for  verisimilitude...duder.

Play it then watch the ER. I'm playing P4 again while I'm watching it (up to episode 51), and as much as I've played the game I'm still enjoying the hell out of the ER. You'll be able to appreciate Vinny and Jeff's reactions and commentary, and anticipate when they're coming up to the best parts of the game.

I've watched the Endurance Run, and now I don't feel like I need to play through the game alone... so I'd recommend you play it first, the watch the ER for the commentary. After all, I wouldn't watch a DVD commentary first and watch the movie second.

Please, watch the ER first.  Trust me!  The drama came from the surprises and Vinny and Jeff's reactions along with your potential ones.

Watch the ER. It's like the game but more funniness!

Also, sometimes Jeff and Vinny will do or choose stuff you never did or wouldn't have thought to do when you played it. Like the nurse.
"You spent some time with Sayoko..." LOL, I always went with options that scored SL notes so I never actually accepted her advances.