The single is a great cage. For me the only drawback is that it is a bit low for accessing the rats. It is a good sized cage and easy to access for cleaning. My preference is for the double, but there is nothing wrong with the single.
The critter nation is a wonderful cage, if you can afford it, go with double though. It isn't that much more and a lot cheaper to a double than to add on later. Rats are kinda addicting. Two can easily turn into 6+
I'd say Critter Nation is worth the money. I have the double, but I feel I'd say the same thing about the single.
The fact that the doors open all the way is really nice. It helps you corner shy, skittish rats, though hopefully any new rats you get wouldn't stay skittish for long. The open doors help with cleaning. You can easily put your head and shoulders into the cage so you can wipe down the bars. The shelves come out easily (warning: though the cage is 3' wide, you'll need 6' clearance to open both doors, and you need to open both doors to get the big shelf out).
The plastic shelves are durable, though my rats had a habit of chewing on them, so they're a bit jagged. Still not bad enough to order replacements after a year.
The bars are close enough to keep even the tiniest rat from escaping, and rats like to climb them. One thing I'd warn about is that these bars are closer together than the starter cage I had. So, the water bottles I have feel like they're pushing against the bars. The space pod tabs I have are a little harder to slide into place. And the wheel I originally got for my baby rats does not fit through the bars. In general, this isn't a big deal, but if you have a lot of cage fixtures, you may find something to be tight.
I love that the cage is on casters. It helps me when I clean out the cage because I cannot open one door all the way in its usual spot. When it comes time to cleaning, I roll the cage out to the middle of the room and clean it. People who have driveways and hoses can take the cage outside for a spray-down.
The metal cage keeps the rats in. Plastic-bottomed cages are usually good, but one of my friends had a situation where her rats figured out how to chew through the bottom. She tried patching the hole, but they can't unremember that, so they chewed through that too.
There are a few spots where I see rust. The cage needs to be kept clean. Just over the weekend, we dismantled the entire cage and washed each component. It's amazing where urine can get to and just collect. But it's pretty easy to dismantle. I did have to whack it a few times with a rubber mallet to loosen the cage though.
I cannot compare this to other cages. I've heard good things about some of them, so I'm sure they are worth the money too. But I can wholeheartedly endorse the Critter Nation. These guys put a lot of thought into their product.
I personally think that the CN is the best cage you can get. The two biggest pros are the cabinet style doors and the removable shelves which make it so easy to clean (as previously mentioned). I really haven't found any cons besides it's a bit pricey.
I only have two boys, so the single is plenty of room for them, but like others have said if you can, get the double. I honestly wouldn't recommend any other cage but that's just me. It's completely worth the money.