I bought a used 2009 Shamrock 21ss. I love the layout, and it seams to be better built than my last Popup which was a Starcraft (supposed to be one of the better built ones... not).
Buying used, you don't know if things were just not taken care of by the PO, or if it's the MFG who never finished the job the right way. All campers that bounce down the road at Hi-way speeds for hours on end, then get setup on levelers or stabilizers for a weekend or more need a LOT of extra attention.
Then many of us let them sit outside all winter in the freezing cold hoping and praying that we winterized it right, hoping that no rodents make it home for the winter, and hoping that the heavy snow & ice don't collapse the roof or find a way in around the vents, skylights and other area's that are hard to seal.
All in all, if it moves, it will need maintenance. However, if you bought it new, it should be good to go for at least the first several camping trips