They're decent. There are several brands out there better than those, but Gravely's are less expensive (at least in my area) due to a few differences here and there (the deck is usually the place to make sure you have the best materials and workmanship, and is usually the one place people don't notice if they've skimped or not).
Here are a few brands to check out
Exmark - EXCELLENT machines, top quality decks and patented roller system that lays down those nice stripes better than anything else I've used. If I were to buy something, this is what I'd shoot for. Somewhat pricey, but worth it, IMO
Toro's - better than average materials used for their machines, fairly easy to get parts for, I'm just not sure they're worth the money they charge for 'em, perhaps they charge that much BECAUSE it's easy to get parts for 'em.
Dixie Chopper - there isn't a FASTER mower made today. Depending on your weight, these things go between 15-18 mph. Parts ALWAYS seem to be an issue with these, which is why we got rid of them. One bright spot is that the belts and drive setup is one of the best designs on the market, and VERY reliable (Toro and imitators are THE WORST design, IMO)
Skag - I haven't had ANY experience with these, but folks who have, swear by 'em. They seem real similar to Tiger's, again, no experience.
Grasshopper - I don't know if there's an attachment you might want that they DON'T make for a Grasshopper. Similar belt and drive setup to the Dixie Chopper.
John Deere - expensive (in my area) and nothing that jumps out at me, just a decent solid machine. Not certain if I believe they're worth the price they charge, but if you're looking for a new one, their diesel mowers (to me) are smooth as silk and I wouldn't think twice about buying one of THOSE.
HTH