I have an ARE "high cap" (my term) topper on my truck. It has full length side windows on both sides that are top hinged, so they lift out of the way to access the bed. I can get up into the bed and walk, head bowed down, in it. I mostly love it so far, and it has been 8 years with it.
The only issue I had with it was very poor customer service from ARE - the locking bars for the rear window were barely long enough, and when going over bumps a little bit of jostling was enough for the rear window to come loose and open up. The dealer I bought it from paid for long enough locking bars and replaced them... according to the dealer, ARE said they knew about the issue, but it was so rare that they just offered a kit with longer bars - that the customer had to buy!! Maybe, hopefully, ARE's customer service policy has changed from back then.
The biggest issue that I see with truck caps is people getting the "roof height" caps. Then many people take them off in less than a year and put them up on blocks behind their house. Those "low caps" make it very difficult to get to all parts of the bed. A "high cap", although may not be sleek, allows you to get into the bed. Some people add "bed sliders" with low caps... where a sliding shelf is basically the bed of the truck. You can get to stuff, but expensive.
Ladder mounts - similar to what has been said, be sure of what you want. The mounts that just bolt to the cap are usually VERY lightweight due to potential damage to the cap - you don't want to be stacking stuff on those, but a single ladder or kayak should be fine. The heavy duty racks that wrap around and attach to the bed of the truck are great and heavy duty, but depending on the cap design you may run into issues -- with my particular cap, the rack might block my side windows from opening depending on the model.
You have an amazing amount of options for caps, especially if you are buying new. And... if you haven't looked before, you may be surprised at how expensive they can be when new.