Need Advice: Of Games and their Sequels/Prequels

Should I buy the first part of a game before playing the sequel? Examples:

1. Should I play Mass Effect 1 before playing Mass Effect 2?

2. Should I play Left For Dead 1 before playing Left For Dead 2?

I'm not just taking into consideration of following the storyline, I'm also considering the cost in terms of time, money, and effort in finishing the first part of the game before playing the second part.

I have SUPREME COMMANDER and SUPREME COMMANDER Forged Alliance to finish before planning to get SC2.

I have to finish Dawn of War: Dark Crusade (just the Orc Campaign is left unfinished) and Dawn of War: Soul Storm to finish before planning to buy DOW2.

A few years back, I got X2 Reunion. I didn't know about the first part of the game, and when I started playing X2 Reunion, neither did I care about the first part. However, now that X3 Terran Conflict is out, I feel like I should have waited for X3 to come out instead of getting X2 first.

I need your advice people.

It really depends on the game. For those examples, Mass Effect is a game that you should play before the second one, but L4D and L4D2 are so similiar that you can do either or (There really wasn't a 'story' that you'd miss).

I totally agree with Legendary on his take with Mass Effect 1 and 2 and L4d1 and L4d2. I mean Mass Effect I definitely recommend getting 1 and 2 first off its great that you can import your characters from the first to the second also they both have good stories and the first Mass Effect you can easily snag for $20. L4D on the other hand has virtually no story so if you want that gory zombie killing game play I say you just get the 2nd especially since its majorly on sale atm if you go to amazon. 26.99 as of today. Compared to the 59.99 price tag it still carries in most stores.

Interesting post about the l4d expansion, however I don't see much coming of that storyline...anyway if I were to make a scale of 1-10 about games I recommend playing the first before the second I would put the Mass Effect series somewhere around 8-9 the L4d series somewhere around 1-2 (from my standpoint its very unnecessary to play the first before the second)and then just to add some perspective on the scale put the Assassin's Creed series somewhere like a 5, the Bioshock series like a 5-6 and then the Modern Warfare first and second at like a 3-4ish. Anyway I have no idea if this post helps but if you've played any of those games it may give you an idea. Of course all of this is just my opinion.

My gosh...excellent replies you guys. Your input has made my OC mind a bit more decisive.

Thanks a lot.

Am making a new thread based on this one.

Another game that I should have included: Bioshock.

Should I play Bioshock 1 before playing Bioshock 2?