I use a medium ALICE every day as part of my equipment at work. It rides in the cargo area of my vehicle for potential incidents that require 24 hours, and can either be used as it is loaded or I can grab little bits and pieces as needed. Or when not working I can grab it and just head out for the night.
The aluminum frame and USGI kidney pad are useable, but not comfortable. They were designed to be universal, and around an average height of 5'8" persons. If you're taller than that, it just may not fit your torso right, and while you can carry it, it might not be comfortable over the long run. The frame can pop rivets under stress and the metal buckles in the older kidney pads really add a lot of weight for not a lot of support. There are aftermarket aluminum frames by companies such as Tactical Tailor that use welds instead of rivets and these are a much better quality. However, the aluminum frame also sits away from your back, creating a bit of a gap in the lower back. Some people like this. It provides a space for storage of a hydration bladder, or a foam pad, or a poncho. It also aided in ventilation. However, it also made the center of gravity further away from ones back, and not everyone likes that. USGI ALICE shoulder straps, especially the more padded enhanced ones, are just basic straps that can still serve the function comfortably. One can add a sternum strap to them, but they don't have anything like load lifters. They are just basic straps.
The Hellcat conversion is a very comfortable way to give these packs a new life. It basically uses an ALICE or aftermarket 1606 frame (more on this in a bit.) along with a better suspension from the MOLLE/PALS system. The MOLLE padded ruck shoulder straps are more of a yoke method and allow for some personal adjustment for torso height and allow for a little bit of load lifter use, albeit not as efficient as on a backpacking pack. However, the ENHANCED shoulder straps do not have the ability to attach the same way. The MOLLE padded waist belt (as well as the US Marine Corps FILBE equipment.) provides a much better support, and other aftermarket options from Fireforce Tactical or Platatac are even better still. USMC FILBE shoulder straps and waist belts can be compatible with ALICE upgrades like the Hellcat.
The company that makes the synthetic frames for the USGI MOLLE rucks is Downeast Innovations Inc (DEI). The DEI 1603 frame had some teething problems, but the version they've had for a while now is a pretty robust frame. But it's a huge frame, like a packboard. Massive. The 1606 was developed for airborne use, and people started noticing it could be made to work in the ALICE rucks.
The 1606MC frame with MOLLE or FILBE suspension breathes an awesome life into ALICE packs. It's not lightweight like a backpacking pack, but it still can be comfortable. My empty pack weight in this configuration is about 7.5 pounds. If I add the MOLLE sleep system that the actual Hellcat conversion uses, my weight is over 8.5 pounds for the empty pack. That is the negative to it. So it comes down to balancing out what you need out of a pack setup. I find the medium the perfect size for pretty basic outings. I put my sleep system in another bag and secure it to the underside of the pack and the frame. If I need additional capacity, MOLLE or FILBE sustainment pouches provide an additional 5 liters each and can be attached to the sides of the pack. A haversack can be stored on the top of the lid. It's got options and possibilities, albeit at a cost of weight. Again, it comes down to what you want or need. If you need to shave weight for a base weight, ALICE probably isn't the best option. But if you need something a bit more durable, or if you just prefer the vintage looks, a medium ALICE can work nicely.