Not really that good. If you look on the bottom of Better Bottles, you'll find the recycle code is a 1, meaning that it is a higher quality plastic that doesn't allow a lot of oxygen permeation. Your water bottle is likely a 6 or 7 so it is an inferior plastic. You can use it, and some do without problems, but most serious brewers shy away from them.
I use them and they work just fine, but the ones I have ARE #1 PET plastic. They come from Dahlonega Spring Water from Georgia. They are identical to Better Bottles except that they are tinted blue. You can still see the fermentation going on inside.
Be careful though, most of them are not #1. They are #7 which is not good. They aren't impervious to oxygen and might impart off flavors from the plastic.
If you do use water bottles, the hole in the cap is 5/8" so you can use a 5/8" outside diameter vinyl hose for a blowoff tube. It works great. That's what I use and never have any problems with it.
They are very easy to clean too. Just put in a couple of scoops of Oxygen cleaner (I use Sun Cleaner) and fill with hot water and leave for an hour or two. The bottle will clean itself and all you have to do is rinse.
The only downside is you can't make a full 5 gallon batch in them because there's no headspace for the krausen. You have to cut your batches to about 4 1/2 gallons.
This question gets asked all the time... this is the second time I've seen it in the last week I think. There are plenty of existing opinions if you search. The general consensus is that it's not a good idea unless you get #1 (PETE) plastic containers (though a few people choose to use the non-PETE ones) and even then, they're too small to make suitable primaries. For starters, in the wiki: http://homebrewtalk.com/wiki/index.php/Carboy