I owned a couple of 91's back in the 80's/90's. Sold both around 2000 for right around $2000 each. Made a bunch more on the accessories, especially the HK claw mounts.
Both of mine were accurate and reliable, as were all the HK's Ive owned. They really arent any heavier than any of the other similar rifles of the era, but their balance is a little different, and the weight slightly more forward.
Their recoil impulse is a little sharper due to the delayed blowback/roller lock system, especially when compared to some of the adjustable gas guns, like the FAL's.
If you dont have a port buffer on them, they do tend to beat the brass up pretty good. With the buffer, there is little damage and the brass is readily reloadable. The fluted chamber just leaves a lot of soot, and some minor scratches. I did find that I was only getting about 6 reloads out of the brass fired in them, compared to 10+ with my M1's/M1A's, which I always attributed to the force of extraction with the delayed blow back system.
The sights and triggers tend to confound many US shooters. The triggers really arent bad once you get used to them, and you dont need to modify them to get good results. Companys like Williams do trigger jobs on them to make them more palpable to trigger sensitive shooters, but they also tend to limit the guns usefulness for anything but target shooting.
I always felt the G3 family had the best combat iron sights of any of them. Once zeroed, they are simple and fast to use. The big "notch" is the combat sight and is used with the front "globe" for CQB, the smaller "V" at the bottom and the "post" to 100 yards, the peeps for farther out.
The claw mounts (HK or ARMS) offer a zero repeatable optics mounting that are easily attached and dismounted. The commercial HK mounts with the rings are not the best if you like your head down and forward. The Germans seem to thing holding your head high and rearward is the way to go. I always found the ARMS flat top mounts to be more versatile and usable. My favorite scope was a Beeman SS-3 1.5x4 on an ARMS mount. The whole thing would fit into a pocket on your utilities or field jacket. With an A2 stock on the gun, you dont need a cheek piece, with reasonable optics, and simply have to slide your cheek up the stocks "ramp" a little for a decent cheek weld.
They offered a couple of versions of folding bipods that clipped to the front handguard. I always found them to be annoying. Most didnt allow vertical adjustment and the gun sat to high when you were prone. When folded, they interfered with your grip on the handguard, and never locked closed tightly, and moved around a lot.
The sling system is what most of the others attempted to copy. I still like theirs the best of most of the others Ive tried.
The biggest thing the HK's had going for them, especially when they were current, was the fact they were "modular", and easily reconfigured. That, and being able to quickly take optics. Something most guns of the age were not. Thanks to them, their early innovations got the juices flowing with the other guns, and were in a better place today.
The downsides for many shooters, were the placement and operation of the charging handle, and the lack of a bolt hold open after the last round. Both are fairly easily overcome with a little practice and familiarization. US shooters seem to be more hung up on the BHO feature. I never thought it was really a big deal, as when the gun stops running, you just reload it, like you do anything else. The G3's/select fire versions do have an additional "paddle" mag release, which the commercial 90 series normally dont. They do make mag changes a bit simpler, but once you figure out how to work the button on the right side, its really not a big deal if you dont have one.