Yeah, the Germans wanted the FAL (G1), but the G3 wasn't a bad consolation prize.
I want a HK91 just because I toted one in the service, but it's not a magic sword...no rifle is perfect.
The G3 is very rugged, more so than the FAL. It's easily scoped via claw mount. The iron sights are very good, once you learn how to use them properly. Recoil is pretty mild for a full-power battle rifle. Ergonomics are decent, field-stripping is simple (even the bolt group). The G3 is probably the most reliable battle rifle ever issued. Accuracy is on par with the FAL, and the G3 has no gas system that needs to be adjusted. Magazines are very cheap, especially since the Germans switched to G36s and dumped all their surplus G3 mags on the market.
On the downside, G3/HK91s are heavy. They lack a bolt hold-open feature, which is a major annoyance, especially since cocking a G3 requires a lot more force than cocking a FAL or M14. (When you pull back the charging handle, you force the roller lock out of engagement with the receiver, which takes more effort than simply retracting a rotating head bolt.) The hole in the G3 charging handle is intended for clipping the carry sling into it to assist in working the bolt manually, if that tells you something. We used to use that feature a lot when shooting blanks, since they have a tendency to cycle unreliably.
Magazine changes are easy with the AK-style paddle release on a G3, but somewhat awkward and unergonomic on a HK91, which only has the button release on the right side of the magazine well.
All in all, it's a fine, sturdy and powerful rifle. I wouldn't drop $2000 on one, though...that kind of money can get you an AR *and* a DSA FAL. Their price has been blown out of proportion by the ban and the HK logo on the magwell, but they're not head and shoulders above the rest in any way.