I got both for $1 around New Years' last year, and I liked 'em both okay. Opposing Force is, as stated, very hard (I admit I used cheats near the end just to be able to reach the ending

). Blue Shift is almost like one of the HL2 "Episodes" length-wise (shorter even? Probably) and doesn't introduce any new enemies or weapons, while Opposing Force has plenty of both. It does, however, help to give some insight into how Barney Calhoun survived the Black Mesa ordeal, so if you're into that sort of background story complete-ness, it's pretty good.
Not sure if I'll ever replay either game, but for as cheap as they are right now it's quite worth it.
EDIT: Whoa, forgot something MAJOR you need to know about Opposing Force: There is a door about 2/3 of the way through the game which obstructs your entire path and must be opened in order to progress, but opening it
crashes the game. Apparently Valve no longer updates these games so you're stuck. There are, however, a number of videos out there of how to carefully noclip to the next non-crashing point in the game (noclipping through the door also crashes it as it's having whatever is behind it on-screen that actually causes the wreck).
Just something to keep in mind when you buy OF.