The Metamorph has been a subject of a lot of debate on the boards, with the general consensus being that it seemed really neat but X was better. That aside everyone also seems to agree that its a neat archetype if less powerful than baseline alchemist.
With that in mind i kind of wanted to start a thread discussing the class, putting it under a microscope and doing a little group optimization on it.
My take is, I think it works best built for resilience. The non bomb/infusion discoveries you can take are all heavily geared toward making you tougher. As i was looking at what was available I kind of sorted them into "discovery Modules".
The first one, and the one i think absolutely has to be taken is Tumor Familiar with the Protector familiar archetype. This module actually adds up to an awful lot of defense..+2 AC against a number of attacks, and eventually a pool of 50% bonus hit points that has fast healing 5. You'll probably never want to separate from your familiar. I expect either + will save or + initiative familiars will be the best choice here.
I'm having trouble ranking the other modules either because they require feat interaction, seem close in power to me, require a "tax" choice, or dont come online til end game so the rest will be in no particular order.
Preserve Organs and Mummification. Its a pretty standard alchemist choice but cold immune simply narrows the elemental resistances you might worry about and stifles some of the save or suck that might bypass your crazy physical resilience. The biggest issue is the tax option of taking 1 rank of preserve organs...you get it naturally and it doesnt stack. This might make it an unattractive option to many people.
Enhance Potion, Extend Potion, Eternal Potion Powerful, and even kind of useful at low to mid levels since a natural attack heavy class such as this gets strong benefits from most buffs out there. The problem is it doesn't really hit its stride until eternal potion at 16th, and without this chain you don't really have to give much of a crap about your int stat. So all in all, pretty stat dependent and doesn't really come online fully until later in the campaign. A class that can shapechange into a troll and get regen DOES however have a pretty good reason to want permanent of level Resist Energy.
The Mutagen discoveries Greater and Grand go without saying, you want to take them, you cant til 12 and 16.
This one is weird and requires some party cooperation and expectation of DM variance on the subject matter, but unless I missed an errata, the archetype loses alchemy and a bunch of things, but doesn't lose Brew Potion in theory then, with some party cooperation to provide the spell you could use Brew potion and Dilution to kick start your ability to use the feat. With the spellcaster providing the initial cast, and the magic item (second potion from dilution) as a seed to continue brewing potions in order to ensure you have the buffs you'll want available when you want to use them. This discovery module works absolutely best if you're doing the potion chain as well, but I think its also the one that is most likely to need run by your GM and party before committing to it. Might not work, might have missed an errata where the archetype loses brew potion as well.
Here is one that I think might be pretty race restrictive due to the feat requirements and probably wont come online until quite late but is really kind of impressive to me. Spontaneous Healing,Lingering Spirit,Endurance,Diehard,Deathless Initiate, Ironhide, and Deathless Master Long story short, you've got a super ferocity where you don't lose hit points and from 0 to negative con+10 you fight at +2 to hit and damage. It wouldn't be my first choice of a build but plays strongly into the theme of a tough character...or antagonist. Plus to be honest, Spontaneous healing is a must have for this build, it stacks with your future regeneration, and between SH, regeneration, and your tumor familar's fast healing, 15 hit points of damage a round can disappear when you need them to.
Beyond that, most of the rest of your options stand alone, without much synergy. Nauseating flesh is probably the best of these for survivability increases, nauseated is pretty heinous, but it, like the potion trees is really going to want you to invest in INT so the save might be failed. Elixir of Life is always good, if expensive.