I use one everytime I work out at the gym and I also have one at home that I use when I'm doing other stuff. Suffice to say they are freaking awesome and one of the best things to hit gyms and exercising equipment in a very long time. I don't use the one with attachments because you can use other things for that. Honestly I just bought mine at Target for $15-20. You can order off the internet if you want but most large chain stores have them. I bought a generic one and it's lasted over 2 years now.
Using it at home can basically wipeout having to buy a bench or a movable bench that can do incline because you can use different seated positions to get the right angle you need. Doing seated exercises helps with stability and core muscles as well as push-ups off the ball and ab exercises. You can do so many things off the ball it's awesome.
Can't recommend one enough.