I have just finished Halo 2 and Half-Life 2 today after taking a break from both games. There may be spoilers in the rest of this post, so this is warning for anyone still playing them.
I didn't realise I was so close to the end of Halo 2 when I stopped playing before. The comments that had been flying around about the ending being a load of crap were right. Unfortunately, I think about the same for Half-Life 2. In fact, that whole game has too great a sense of mystery about it for my liking... But I still love it. Reading around on the Internet has put a few of the puzzle pieces together and now I just have another 6 years to wait for Half-Life 3.
In my opinion though, it is a shame that games are left far too wide open for blatant sequels. I would rather have some sort of closure to the current events and have the ambiguous endings of the current games be the start of the next game if there ever is one. Why not even replay the final battle as the intro? Which is worse though, tacking on a cheap plot extension with little explanation just to produce another sequel, (making more money but raping a successful franchise, like too many movies,) since things were too tightly wrapped up, or leave it open and confusing? But then I still want to buy the next installments for both titles... What can I say? Consumer whore.
Oh well, multiplayer is here now for HL2. I'll get X-Box Live! to play Halo 2 when I get back here after Christmas. HL2 should have some interesting mods released for it eventually. I like the sound of one - Resistance Vs Combine. An obvious, but potentially excellent team based shooter.