I recently signed my two girls up with VPI. Thankfully I haven't had to use it yet so I can't speak to how painless the claims process is but it gives me some piece of mind. If you do a search here on pet insurance you'll find several companies that people have their cats at. Not that many people have posted about filing claims... cat forum folks take good care of their fur kids!
As far as what to watch out for when you compare plans... obiviously what's covered, how they deal with pre existing conditions or don't, limits on actual pay out per incident and/or year, deductibles, any age limits when they will drop your pet, and whether or not you can go to any vet you want to. I believe all the plans I looked at you had to front the money and the insurance company reimbursed you. You mentioned that your cat is a senior so you probably have several other issues to consider that I didn't.
I asked my vet if she had any experience with any of the three companies I was considering and she said that she's filled out claim forms occasionally. She said that no one's complained to her about the insurance company but of course if you had a problem you would be complaining to the insurance company not the vet so I dont' know that that's very helpful. VPI is the company that my vet has pamphlets for, again I wouldn't say they endorse the company necessarily aside from confirming that it's a legitimate company.
Like any insurance you need to read the fine print and it's mind boggling at times. I guess this long winded post probably isn't much help but like I said, do a search and see what everyone else had to say.