I'd buy it for PC because of online multiplayer. I haven't played it in a long time but it's Reach without semi-regenerating hp.

I think the core gameplay of Halo: Combat Evolved still holds up well but I can't guarantee if you will, you didn't like Halo 2 and 3 so maybe the core gameplay of the games is not what interests you. I can't give you an exact figure for the average play-through time of the game but rest assured, although the campaign is a good length I wouldn't say it's longer than most Halo campaigns. If you just want the story then you could read The Flood although I get the impression most Halo fans consider The Flood a rather poor adaptation of the game. Still, you do seem to be very interested and if you have the 1200 points then I say go for it and try it out, I can't guarantee you'll like it without knowing exactly what you like about Reach but from an objective standpoint (if there is such a thing) I'd say most people wouldn't consider the game too out-dated. Even you find the game is not for you and don't want to read the book you could always quickly brush up on the plot at Halopedia. Suffice to say Combat Evolved/The Flood does act as a pretty essential bridge in the story to know what's going on at the start of First Strike.
 
EDIT: @Conker: That would be Sgt. Johnson.

I can't speak for everyone, but I think Halo CE still plays fantastically.  It doesn't feel "old" like alot of games do.  I have played current generation games that I felt played "older" than Halo CE does.  It doesn't look to shabby either because of the art design.   
 
As for length, probably 8-10 hours.

Certainly, it's considered by many fans to have the best campaign and story within the series. When you do get the chance to play it, play it on the Heroic difficulty first. The game is filled with dazzling set-pieces, mission variety, superbly paced vehicle segments, excellent enemy A.I., and vast outdoor levels. 
 
The game does have it's flaws though. The indoor levels are mostly copy-and-pasted throughout, and The Library is one of the most mind numbing experiences I'v had in a video game. But the gameplay makes up for those flaws. 
 
It also has the best pistol in video gaming ever.

Its dirt cheap for PC, might aswell. 

The controls in halo CE are still better than a lot of games that come out now. The game holds up awesomely, it's one of the only old games that I go back and play again and again. Be sure to play in co-op, it's unbelievably fun.

Wait a sec. It's $15 for that game? That's a lot for a game that did not age well. 

" Wait a sec. It's $15 for that game? That's a lot for a game that did not age well.  "

" @ch13696 said:

" Wait a sec. It's $15 for that game? That's a lot for a game that did not age well.  "

I know. I mean, it's dynamic A.I., sandbox combat, smooth scripting, and vehicular combat, doesn't hold a candle compared to the jdozens of military, "aim down your sights" shooters where all you do is shoot through wave after wave of brainless baddies from one overly-scripted set-piece to another. Halo is so generic compared to those games. I mean colors? Pfft, what games need that anymore? "

" @Cegoraph said:

" @ch13696 said:

" Wait a sec. It's $15 for that game? That's a lot for a game that did not age well.  "

I know. I mean, it's dynamic A.I., sandbox combat, smooth scripting, and vehicular combat, doesn't hold a candle compared to the jdozens of military, "aim down your sights" shooters where all you do is shoot through wave after wave of brainless baddies from one overly-scripted set-piece to another. Halo is so generic compared to those games. I mean colors? Pfft, what games need that anymore? "

Ok, so let me rephrase that. For a game with no multiplayer and the only replay value is trying other difficulties, it's not worth the $15. Like most people are saying, get the PC version. "

" @ch13696 said:

" @Cegoraph said:

" @ch13696 said:

" Wait a sec. It's $15 for that game? That's a lot for a game that did not age well.  "

I know. I mean, it's dynamic A.I., sandbox combat, smooth scripting, and vehicular combat, doesn't hold a candle compared to the jdozens of military, "aim down your sights" shooters where all you do is shoot through wave after wave of brainless baddies from one overly-scripted set-piece to another. Halo is so generic compared to those games. I mean colors? Pfft, what games need that anymore? "

Ok, so let me rephrase that. For a game with no multiplayer and the only replay value is trying other difficulties, it's not worth the $15. Like most people are saying, get the PC version. "

Have you ever played split-screen on GoldenEye with your friends? Have you ever heard of the phrase "co-op" or "lan-parties"? "

" @Cegoraph said:

" @ch13696 said:

" @Cegoraph said:

" @ch13696 said:

" Wait a sec. It's $15 for that game? That's a lot for a game that did not age well.  "

I know. I mean, it's dynamic A.I., sandbox combat, smooth scripting, and vehicular combat, doesn't hold a candle compared to the jdozens of military, "aim down your sights" shooters where all you do is shoot through wave after wave of brainless baddies from one overly-scripted set-piece to another. Halo is so generic compared to those games. I mean colors? Pfft, what games need that anymore? "

Ok, so let me rephrase that. For a game with no multiplayer and the only replay value is trying other difficulties, it's not worth the $15. Like most people are saying, get the PC version. "

Have you ever played split-screen on GoldenEye with your friends? Have you ever heard of the phrase "co-op" or "lan-parties"? "

Ok fine. You win. But, can you even do that with the version on 360? I haven't seen or heard about anyone trying that. If you can't, then do you think it's still worth it?  EDIT: I'm pretty positive split-screen will work, but not sure about the other modes. "