What's your opinion on using walkthroughs?

I don't use them, unless they're for a hard achievement.

i generally play through a game once without help, then replay with a walkthrough to see if i missed anything good

I try to avoid using them. Basically only when I get stuck or when I go for some of the harder achievements.
And FEZ, cause you simply need one to get everything out of it.

I use them, primarily because sometimes game design runs counter to natural playstyle. It's not too uncommon for a game with a story to urge you blatantly towards... lets say a left turn in a junction, to continue an urgent part of a story, but then reward all players who take the right turn and spend 10 minutes wandering around empty, broken rooms by giving them a unique item that can't be found anywhere else, and that serves to slightly enhance the players enjoyment of the game (or their characters flexibility).

Sometimes because of that kind of design, I use guides to ensure I'm not actually cheating myself out of a slightly better playthrough based on something arbitrary. Though stopping to refer to guides does break up the game flow, it doesn't break it as much as spending ages combing every corner of every optional anti-route and re-checking every NPC for new dialogue options and missions, y'know? It doesn't kill a game, but it can be a grumblesome moment when you complete a game, then stumble on a video of someone going all "OMG THIS WEAPON MOD / SUB-MISSION IS THE SINGLE MOST DEFINING POINT IN THE GAME, IT ALTERS HOW ALL THE CHARACTERS APPROACH YOU AND blahblahblah", and all because you decided to stay fully immersed rather than meta-game.

For adventure games, I never refer to walkthroughs, but if I'm stuck long enough (days), I'll ask for a hint on forums.

For puzzle games, I use them after being stuck for a few hours, but sometimes I just sleep on it instead, play another 1 or 2 hours, and only then consult a guide.

For action games, it usually takes only somewhere between 5 and 10 retries for me to give up and see if I'm doing something wrong.

Oh, and I use guides instantly after finishing the main parts to find 100% collectibles in some games, like the feathers in AC2 (and yeah, I know that's ridiculous).

I use them a lot for point and click adventures, don't have time to waste on some insane, illogical puzzle.

As for other games, I usually only use them on my second play to get all the secrets.

I try to avid using it... but sometimes when I'm stucked to long in some point without an apparent logical wayout... ;)

i always have guides running side by side

if the game is interesting enough I will never use a walkthrough regardless of how hard it is (for example: portal 2)

When I get stuck and I feel like the frustration caused by the thing preventing me from getting further outweighs the satisfaction of beating it on my own.
The most common reason for me using a waklthrough is because I'm playing some adventure game that has a serious case of backwards logick.

Hmm, my big regret with looking up things is Dark Souls, thought it was just an rpg you play once and forget, so better I look things up to get all the fancy loot for my character. :(

Didn't realise it's the sort of game you reach the end, and then immediately start playing again. :) Would have enjoyed trying to discover things on my own with multiple playthroughs, instead of reading about the secret hidden here, the good item there, how you can join a covenant by doing X, etc...

So anyone buying/winning Dark Souls in the next sale, just get the pc version fixes working and play. Don't watch any videos, read any wikis, spoil yourself at all...

I try to complete the game first without using any walkthrough or guide and once i'm done i check out for curiosities or easter eggs i mighy have missed on my first playthrough.

Essential when trying for 100% completion and all endings and secrets, especially for guide dang it elements.

Depends on the game. I use the binding of isaac wiki because even after 140 hours I can't remember what every item does. I used it in dark souls because I had no idea where to go to find the 4 kings. For platformers like SMB if I've died 100 times on the same level or while doing rocket jump maps in TF2 if I'm stuck in the same jump for 45 minutes, I'll probably check a video to see how people do it. Other than that, I pretty much don't use them. I'm not a completionist, so I don't care if I miss something.

i avoid them, but after i got stuck on half life 1 for a few good hours i looked up a video and turns out i had a bug that prevent me from going on, i was so annoyed, not only i had to load an earlier save file to keep without the bug but i had also wasted hours by solving nothing.

Use them.. but only if I have been stuck for a long time at the same place (generally like 1 hr or so) or doing the extra playthrough for extra items, collectibles or Hard Achievements.
I believe they make the game interesting by providing extra options that you have not found out by yourselves but only if used at the right time

If you over do it its kinda takes the fun out of the game.

But some rare times they are essential to keep the fun going...

before i was like damn it's like cheating and i've never done that, and now i'm like, if i'm stuck for too long and i don't care a lot i check solution for one problem and just push the game forward

If I'm not having fun in the first place, I'm much more likely to hit up a walkthrough, but mostly I avoid them.

One reason I'll eventually cave to a walkthrough though (especially if a night of sleep didn't help), is that you never know if the issue you're having is due to a game bug, which has happened to me on more than one occasion, even on some of my favorite games.

And if I'm playing an adventure/puzzle game, I'll always look up a "UHS" (Universal Hint System) guide before looking up a walkthrough. A lot of the fun solving some games in my past (Zelda for gameboy, The Neverhood) was in cooperating with close friends "hey man, how did you get past...?!" / "where do you find...?!", and I think of UHS as a good extension of that when I don't have anyone else playing the same game.

If other people want to ruin their own experience, I'm not gonna stop them.

As for me, ima keep trying that shit til I manage :D

They are helpful for 100%ing games

I think it is the player's choice. I personally won't use them unless I hate my life.

I'm addicted to walkthroughs. I'm a completionist/trophy whore/achievement hunter and I don't usually have the patience to play the same game twice just to get/achieve something that I missed in the last hour of the game, so I always look for a walkthrough even before I start to play the game.

I play games for fun. Most of the time, I find it's more fun to figure something out on my own, but if it gets to the point where I'm definitely not having fun anymore I either set the game aside or go grab a walkthrough to advance just beyond the point I'm stuck on (and possibly repeat the process if I get stuck again later).