I have used them a bit, and I say "meh"... There are pros and cons for both and I think it just comes down to personal preference.
In my experience, yes, they are kind of mediocre in cold weather. This particular lineloc doesn't do a great job gripping line when it gets wet and frozen. They're OK but kind of slippy and I never really trust them in these conditions even though I've never had a real problem. I think the triangle-shaped ones are supposed to be better but I don't have any experience with those.
They will do a fine job 90% of the time. And the 10% of the time when they kind of struggle they should still be able to get the job done. And if they don't, well, then you can just tie knots. There is no question that linelocs are more convenient and easier, and will get the job done adequately.
On the other hand, knots will hold MUCH better and be more versatile. Not having a loop already tied in the end of your line makes it easier to tie off around a tree or whatever you want. My favorite thing about knots is that they make it much easier to put a lot of tension on the line. I would really recommend not using fixed loops, and instead learning the trucker hitch system Skurka talks about. It's really not as hard as it looks at first - you aren't even really tying knots per se. This system gives you a mechanical advantage and makes it very easy to get a super taut pitch, and once you tie them off they hold extremely securely with not even a threat of slipping.
Using a tautline hitch makes adjusting easier but I would stay away from that knot. It has the same problems as the linelocs with not holding all that well especially when the line is frozen, and you also lose the mechanical advantage so it can be more annoying to put tension on.
So basically my opinion is that the linelocs are easier and more convenient and will get the job done, but using a trucker hitch type of system is "better". Whether the convenience of the linelocs outweighs the advantages of the trucker hitch syetem is a personal choice.