SummaryThat's what an expansion should be!
The GoodThe new weapons are very neat. The Displacer and the Barnacle don't see the use very often, but their concept is very nice and original. The new enemies are well thought-out, but it's a pity that they aren't as original as the weapons.
I really loved the level design. (A nice feature is being able to visit some of the areas you visited back in Half-Life!) The authors made great use of the HL engine, the puzzles are as good, if not better as in Half-Life. There are some new gameplay elements added: most interesting are the climbable ropes. They don't appear very often, but for a game made in 1999, their physics are good; you can even realistically swing Tarzan-style on them.
The graphics are quite good and the gameworld is as detailed as in Half-Life.
The "Black Ops" soldiers you fight in the game seem to be more intelligent than the soldiers you fought back in Half-Life. I always thought the HL soldiers' AI was a little overhyped, but the Black Ops appear to be really smart.
The BadAfter getting used to the HL's crowbar, the two melee weapons available in OF are a little inequal. The wrench is strong, but too slow; the knife is fast, but too weak.
The game is harder than Half-Life, mainly due to the new enemies (which are much stronger and more dangerous than their HL brethren), but also because there seem to be very little batteries or HEV chargers in the game, so your armor will be 0 most of the time - and when this is the case, the enemies' attacks are really harmful. Likely the most dangerous enemies are the Shock Troopers, who fire really fast and are hard to take down.
Controlling your soldier buddies can get tiresome at times. Sometimes they decide not to follow you for no particular reason and you have to uncontrol/control them several times.
An annoying bug is getting stuck in elevators. It happened a few times to me, and the only way is to reload the game and jump constantly while riding on the elevator.
The Bottom LineBut all in all, Opposing Force is an excellent expansion, and a worthy addition to the Half-Life universe. If you liked Half-Life but didn't yet play Opposing Force, do yourself a favor and return to Black Mesa as corporal Shephard.