I have a Rival brand one, I think it's 6 quarts. I probably would have gotten a smaller one for myself, but it was a gift. I love that the one I have has different time/heat settings. When the programmed cook time is over it simply switches itself to a "keep warm" setting so I don't have to worry about anything burning or being over-cooked.
I love when I am out all day and get home after work and dinner is there waiting for me. I also use it to cook dried beans and it makes some awesome seitan, as you never have to worry about the seitan boiling and getting that weird sponge-like texture.
There have been a few vegan slow cooker only books. I have heard really good things about The Vegan Slow Cooker by Kathy Hester. I really want to buy it but I'm on a self-imposed cookbook buying ban right now (I have way too many). I do have the Vegetarian Slow Cooker by Robin Robertson, but the recipes aren't that great to be honest (I find most of Robertson's recipes to be really under-seasoned). That being said, the book does have a lot of useful info on using a slow cooker.
I've also just converted conventional soup/stew/curry recipes into slow cooker ones by tossing the stuff in the morning. Most of the time it works out alright.