An excelent, reliable first marker. the late '99s have one of the best stock barrels I've ever seen. There isen't a single external moving part on the 'gun. Mine personally has never had a problem.
I've heard a few horror stories about bad sears and striker springs, but ACI has offered to fix them to all who have warranties. Other than that and the relatively limited upgrade path (which you don't really need), there's nothing bad about them.
Overall, an excelent beginning 'gun at a good price. What sold me, though, was the members of www.f4og.com . If you're thinking about getting one, check their site out. If you do get one, oil it and take it apart once in a while, and you shouldn't have any trouble.