I've been using the Rival 500 for a bit over 2 years now, as my daily mouse, at least 8 hours per day. The rubber grips didn't fall off and there's no sign they will any time soon. They wear off a little bit, but not too much to be a problem. I clean it with isopropyl / rubbing alcohol about once a month or so.
The biggest let down for this mouse is the rubber cable, which is a bit stiff, but I'm using a mouse bungee and then it becomes a non-issue. It's a rather wide and heavy mouse, but I have no problem with that and find it comfortable. I'm using a fingertip grip and my hand is medium in size, I guess, about 18 cm long.
The non-standard buttons on top (11, 12 and 13) are rather stiff to press. The ones on the left side of the left click button (11 and 12) are very difficult to reach without changing your normal hand position/grip. But that's a problem I have with any other mouse that places buttons to the left side of the left click button. (Logitech likes to place them like that on many of their models and that's why they usually are a hard pass for me. I have the G402, which technically has 5 extra non-standard buttons, but only 3 are actually usable for me).
The rightmost button (13), I find it easy to reach with my middle finger, so I'm using it in some games.
The side buttons are less stiff, but not too soft to press them accidentally. The ones in front (05 and 06) are not so easily reachable, for me, like the other four (04, 07, 08 and 09), so they don't get too much use. For the one under the thumb (08), you might need some time to get used to it. I know I didn't use it for quite some time, always forgetting that it is there.
When I was researching for mine, there were some people complaining about some disconnecting problems with the mouse. But I think they were solved with a firmware update, because I haven't had any problems with mine and I did the firmware update when I bought it. There weren't any other firmware updates after that. If you buy one now, it will probably have the latest firmware, but do check, just in case.
I haven't found any better alternative to this mouse yet. Maybe Razer Naga Trinity or Naga Hex v2, but I don't trust or like Razer much, and they are usually on the expensive side. And then there's this cheaper
Trust GXT 155C Caldor, that might be decent.
As for those with 8, 9, 12, 16 buttons in a grid, I don't know. I want to try one, but I don't think they are easily usable. Seems to me like you'll press the wrong button way too often, or have to look at them each time before you press a button. But that cheap Redragon Impact might worth a try.