I put a mutagen on my Vigor skill under Training that has Incineration, Poison, Bleeding, Knockdown, Stun and Freeze +2%. If I find one that I like better can I unequip this one or am I stuck with it?

Also, would you consider this to be a "good" mutagen? And does it matter what skill I attach mutagens to, or do they work independently from one another?

Nope you can't. It's a permanent upgrade

Don't worry too much about mutagens. I know it's tempting to always max your character, but the mutagen system is a real bummer for that.

There are several tiers for mutagens, it's been a few months since I've played it, but I don't think 2% is particularly good. It's best to leave your mutagen slots open till you find rare mutagens. As I say, it's been a while, but I believe that the best mutagens you could get were 'Greater' mutagens which add around 5%. Not much difference so I wouldn't sweat it.

OK Cool. I'll just wait. I like this feature though because the game isn't holding your hand. If you insert one in, you're fucked for the rest of the game. No respec-ing. I love that.

I have found a Rare mutagen alrady, but it's of a Lesser tier so I will just hold off.

@xyzygy: It doesn't matter where do you put which mutagen. What I'm trying to say is, that if you have a health upgrade, it doesn't matter if you put a health mutagen there, or somewhere else.

No, that's why I recommend not applying any until mid chapter 2 where you'll come across some decent ones. Wouldn't restart the game if you already locked a few in though, as I never found them to give me much of an edge.

I'm finding that Greater mutagens were a drop from those Spider-worm creatures, Endrogas or soemthing? I got two Greater Vitality Mutagens from them. So I used one and am saving the other! I heard that they were hard to drop in the original Witcher 2, maybe they changed that.

@PerryVandell: I'm still in Chapter 1 and I've found 2 Greaters already. If I'm not mistaken, Greater is the best right? Lesser, Basic, Greater?

If you're really serious about mutagens, you might even want to consider waiting until you have spent a minimum of six (6) talent points in the Alchemy tree to unlock and upgrade Impregnation to level 2, which will improve the effect(ivenes)s of your mutagens, but only for the ones you 'install' afterwards. Actually, if you're really serious about mutagens you should wait until you unlock Impregnation level 2.

The in-game description for Impregnation may seem somewhat disappointing though, even discouraging: a mere 15% and 35% for levels 1 and 2 respectively. It's not very (mathematically) apparent how they arrived at those numbers or how they apply the percentages to reach the effective result: at level 2 Impregnation effectively doubles the effects of mutagens (that you install afterwards). For example a Greater Vitality mutagen will add +10 to Vitality, but +20 with Impregnation level 2. That's either a 100% increase, or 200% compared to the initial value, or in words: "double(d)". Vitality is probably not 'the best' option though, because there are certainly more cost-effective ways to gain more Vitality than occupying a mutagen slot, it was chosen as example because 10 is an easy number for math; improved values may differ for the various types of mutagens though.

The value and relevance of mutagens and spending (at least) those six points highly depends on your playstyle, experience (is it your first playthrough ever?) and probably difficulty level. On a first playthrough on non-Dark you may not notice the effect of mutagens all that much and you should probably just enjoy the game and its story rather than trying to find 'the best' mutagens.

Mutagens are indeed tiered: Lesser, 'Basic' (no further denomination e.g. Vitality mutagen) and Greater. Some types only (seem to) have a 'Basic' version, e.g. Madness and Concentration.

Also, there is an opportunity to redistribute talent points, but not mutagens late in the game (chapter III), although that's beyond the scope of this topic.

Like @d0rks: mentioned above if you are planing to go into alchemy tree wait untill Impregnation untill slotting any mutagens.
 
Otherwise a rule of thumb is to wait untill you get a high quality mutagen, and just slot it as soon as you get one of greater quality. Madness is a superb mutagen of only one level and should be slotted asap.
The hardest parts of the game are arguably at the start, so waiting for a perfect mutagen mix or build is imo wastefull.
Just slotting the greater drops you get strikes a nice balance between not slotting garbage and waiting too long.
 
(obviously adjust slighly for your build, a full swordsman build will have little use for + spell power mutagens)