I've been using Time Capsule as NAS, but wasn't too impressed for my purpose. I put files on it and allowed multiple computers to share files through it. The transfer rate wasn't fast enough, and for Windows PCs, you can do much better.
I've been using a TC 500 for most of a year, now, and its Time Machine, router, and wireless base station functions work fine. I've watched it work fine, autonomously for months without attention. I've restored items from its contents through the Time Machine GUI, with painless ease.
However...
I would use it as a 1000 mbps Ethernet-based drive but not a wireless-based one, as 802.11 is still just a tad too slow.
I found it difficult to set up, when what I wanted to do was use it to replace an old Airport Extreme, maintaining an existing wireless network with only one other device on it. This should have been easy, but was not, and required a call to Tech Support to get right.
I have, on occasion, had to power cycle it, to correct incomprehensible network stoppages; I have no idea what happened, or why power cycling fixed them.
I have had the backup file system suddenly and for no apparent reason get bollixed up and require repair by Disk Utility.
I have tried to do a restore of its entire contents onto a new machine via the Migration Assistant, and had it fail miserably, with no explanation. Apple was no help in this instance.
I have had space suddenly and for no apparent reason seem to disappear. No repair I can perform will bring it back.
I have gone through the proper Time Machine interface to delete large files I accidentally let get backed up, and had the space they occupied stay missing.
I've had it come close to filling up, at which point TM deleted old backups without notifiying me it was doing so or giving me any options, as it is advertised to do.
I've considered moving its contents to a larger version (i.e. a new a TC 1000), so a set of backups could dcontinue expanding, but have been unable to find a concise, succinct method for doing so.
I have asked Apple about these matters multiple times, both at Genius Bars, and on the phone to Tech Support, but I've never gotten an answer, in spite of repeated promises that I would be contacted by engineers about them.
So, yes, it's a pretty good product. And as long as you use it in utterly mainstream, middle-of-the-road ways, it should work ok. I'd recommend it, with cautions. It may not be the best SINGLE backup mechanism, but it certainly works in a carefully considered backup STRATEGY.
Good luck!
Tom