I had a subscription to Marvel Unlimited a few years ago, before iPads. I kept it for a couple years, found it really good for jumping into Marvel comics (before I'd pretty much only read manga or Sonic the Hedgehog lol). So my experience is a little dated and I've never used the apps themselves, just the browser stuff.
That said, I think it's worth a try. It has a really decent backlog of classics, a surprisingly complete collection of the terrible '90s stuff they did, and it's good if you're looking to read the big events (House of M, Civil War, Secret Invasion, etc). I jumped in after the first Iron Man movie and was able to get relatively current, enough to be able to go to my local shop and pick up on the Big Event at the time (Secret Invasion).
One thing to keep in mind is that there's a 6 month delay between print date and Unlimited date, meaning if Captain Marvel #6 just came out in print, you'll only be able to find Captain Marvel #1 on the app.
One of the major things that got to me though was the reader itself. It was flat out terrible. It was fine for older comics, anything before the 90s because they were all scanned flat. The newer comics had this weird thing going on where the word bubbles were on a different layer than the pictures and the bubbles would end up off screen somewhere. I literally had to search for the bubbles, read them, then scroll back to the image to see who was actually talking. It was absurd.
I remember poking around on the site a few months back to see if they'd fixed it, but as far as I could tell they hadn't.
Now mind you, it might be a different story with the apps. I would definitely try some of the freebies they have to see if you like they way it's done. I can tell you that the moment Unlimited starts using the Comixology tech, I'm going to snap it back up. Marvel uses it for their digital comic store front but not Unlimited for some reason and it's a shame.
Anyway, I guess I've sort of rambled. Long story short, try the free stuff first, or just subscribe for a month and see if it's worth a longer go.