I bought the 590a1, because of its reputation in various LE and Mil services. I've thoroughly enjoyed shooting it and learning how to disassemble and assemble it. Two things though:
1) It is heavy. I've never held a regular 590 to compare it to, but I have held numerous 500's, so perhaps that's apples to oranges. But still, you will notice the weight of a 590a1 compared to most other pump-actions. If you're in shape and can lug that beast around (when loaded) more power to you. If it's just a range gun or emergency defense gun, and will mostly sit in your safe, then that's fine too. This shotgun was designed with heavy-duty military use in mind, which appealed to me. But unless you're using the a1 in that kind of abusive environment (think constantly being banged into metal, dropping, firing lots of rounds in a short time frame) the thicker barrel and metal trigger guard may not be needed.
2) the 20" barrel has got a very long LOP. My arm span is probably about average, and while I can manipulate everything fine, I'd prefer a shorter LOP to help with reloading and mitigating the recoil. I think the 590's and 500's have more flexibility with different LOP's and stocks. The 590a1 used to offer a magpul LOP adjustable stock, but I don't think it's offered anymore.
The a1 is very much a wartime shotgun. It's construction and robustness reminds me of the type of rifles and shotguns I used overseas. If you're into those types of firearms just because, go ahead and get the a1. If you think that getting the a1 is going to be the deciding factor in a life-or-death situation, don't waste your time and money and go get a a 500 or 590 and customize it to your liking. Though to be fair, the a1 isn't that much more expensive than some other pump-actions. I paid ~$520 for mine (minus shipping and FFL costs), and you can easily find Remington and Mossberg pump-actions at that price or even higher.