In my replay of the Half Life series before tackling HL2 EP1/2, I also replayed Opposing Force. Like HL, I still remembered a lot of this expansion. I probably replayed both of them alot back when I had fewer games to play. OpFor was a real good expansion and even though it was not from Valve (It was from Gearbox) it expanded on the HL world well and was fun to play.
Like HL1, the intro lets you look around while you enter Black Mesa. On your journey you run across folks from the first game. This Barney-ish guard sure has let himself go. At least he helps you with some of the invaders, unlike the coward scientists.
A nice part of the expansion is having squad members that join you. While you have limited control, its good to have some extra firepower or help opening doors. I couldn't ever get the medics to heal me though they seemed to help the rest of the team.
Its fun and interesting to re-see parts of Black Mesa from another perspective. Running into old friends is always fun. While I never liked Zen, at least here its brief and just a few jumps as you get zapped back to the main world. Lots of those pesky Black Ops to take out, as they again don't like you.
For some reason I still have the skybox bug. Only other issue I had was I couldn't correctly start the game so I used the console to load the intro and then used the quicksave/load to play the game. Each time I started back up, I had to use the console to load the autosave and then loaded the quicksave to get back to where I was. Wasn't that big of an issue though.
I don't have the Blue Shift expansion so now I go on to HL2. Though guess I could have skipped OpFor since it isn't part of Gordon's tale, but I still find it a fun expansion to a good game.