But you still miss out on significant missions with either course of action (as opposed to random quests that are only found in isolated locations).

I finished Wolfenstein, Doom 1 & 2, Decent 1 & 2, Heretic and Hexen. They were fun. Hexen still rates as having my number 1 favourite weapon.
I also finished Quake3 Arena pretty thoroughly, although it's more fun multi-player. I keep threatening to set a server up again.
I think the last game I finished was DN 3D. I explored every corner of every level, and I think I completed it on the hardest difficulty with no cheats. It took me a very long time to get to that stage though.

I never finished whichever FF it was I had. I never finished Portal because I got bored about 90% the way through. I never finished HL2. I never finished Doom 3. I never finished Resident Evil which I had on Game Cube

I'm currently stuck on DN Forever at a fairly early stage. I may need to check the web for a run-through

Also stuck on Bioshock. Well not stuck, just not made any good progress for ages and then got bored.

I think my Skyrim characters have both been lost due to carelessness with backups. I hadn't gotten very far anyway, so when I finally get a new PC I'll start again.

I've not finished Minecraft, mostly because I don't think the server I'm on actually has "the end". It's not been updated all year! Of course I could do it single player, but I have more fun in MP.

I finish most games, unless they get boring and repetitive.

I've tried Doom 3 a few times, but lose the will to live once I enter Hell. Assassin's Creed got very boring, as did GTA IV (pretty quickly, actually).

What's the deal with Doom 3, it seems to have generated some very bad vibes from what I've read...

It's a marmite kind of game - some simply didn't like how it was staged, expecting it to be the gun n' run shooter of old, but id made the (right) choice to replace stupid sized hordes of stupid enemies with more intense, drawn out, scare moments against stronger, more intelligent enemies. Equally, the map architecture was considerably different, yes, the "originals" focused a lot on cramped spaces, but they also had the big open areas - Doom3 focused on the former - and many (including me) found the replay value wanting (there was/is little room for a change of play styles, and many argued it was almost a shooter-on-rails) - which is why I've not returned, because, despite the game being very very good, I just can't be arsed going back through the first opening stages (although to be fair, both of the Half Life's have slow build-up levels that are bore to replay over and over).

If you've never played it, I would urge you give it a go.

It's a corridor shooter.

It starts of well enough, but there isn't enough variety to hold your interest all the way through (in my opinion).

Cheers folks, I shall be renting the collection come release

That is not surprising considering the bloody game hasn't been completed yet.

Well unless they eventually call the next release HL3 rather than HL2 Episode 3.