A> There's been questions regarding quality control of Marlins since their acquisition by Remington a few years ago. Supposedly, that is being addressed and also supposedly the quality has improved of late. This affects most if not all .308MX's. Not saying that all of them are flaky -- there just seems to be a higher than "normal" incidence of lemons. Just make sure that you check out -- first hand -- a recent manufacture Marlin for quality before buying it.
B> Last I checked, there is 1 manufacturer of factory ammo, that being Hornady. Likewise, Hornady is the only manufacturer of component bullets for reloading, although conventional round or flat nosed bullets can be used. Likewise, conventional spitzer or spire point bullets can be used by either limiting the rifle to 2 rounds (1 in the chamber, 1 in the tube) or by snipping off the lead tips as was previously pointed out. I'd suggest stocking up on brass/factory ammo while it is available, since it makes me very nervous when such a critical component is only available by way of a single vendor.
C> Performance is similar to a Savage 99 in .300 Savage. Not quite as warm as .308 Win, but close enough.
D> If you happen to be a reloader, there is another option that you might want to consider: Get a similar rifle in .30-30 and have it rechambered in .30-30 AI. This has 1 disadvantage in that there is no factory .30-30 AI ammo available at all. Nada. But it is capable of shooting conventional .30-30 factory ammo and in the process you get .30-30 AI brass. Performance is close to .300 Savage. No shortage of reloading components.
E> If you want .300 Savage or .308 Win performance in a lever action without being single sourced on ammo you can always just go with a vertical feed rifle (such as a Savage 99, available used) that is chambered in .300 Savage or .308 Win.
If you really want to learn more about the .308MX, you might want to check out the dedicated sub-forum about them at the Marlin Owners forum:
http://www.marlinowners.com/forums/