I have two, one stock with the .50 and one with a Pac-Nor .45. Many aftermarket parts and plug options available. I hear mixed results with the stock .50 barrels. (I still have the original with the pac-nor, and mine were both shooters) I think they are good guns, and you can always easily customise anything.
No problem getting parts or upgrades from smokelessmz.com and a few things at hankinsrifle.com.
Only real maintenance is changing vent liners in the breach plug when they wear. Vents cost $3-5 depending on brand and are simple to replace. That can be pretty much avoided with a tungsten bushing plug upgrade for around $80-90. Bushings can last over 1000 shots BUT they are about $30 to replace.
OEM composite stocks pretty much suck. Boyds sells laminates fairly cheap and there are a few others that fit too. Boyds will need pillars and bedding to make them a good upgrade. The OEM laminate isnt too bad with just a bedding job.
OEM barrels can be finicky. Aftermarket barrels though are SWEET!!!! Swap out to a 45cal and never look back.