The MEDIUM sized band is a great all-rounder for mobility drills and assistance with gymnastic strength exercises. I use it when I don't want too much resistance for mobility drills or to assist me for planches when (in my case) I want the intensity low. Great for starting out and/or when something like the Deuser Band is too strong for the task in hand.
Like the Rubberbanditz band above, I LOVE the Deuser band for mobility work. They are strong and durable and because of the former will likely take a while to get used to if you are using the for say shoulder mobility work (Shoulder dislocates with a band fresh out the box will let you know where you shoulder are at ;) ). Great for the stronger person and something to aim for for those starting out. PLUS it's German... enough said.
Maybe not the most notorious brand of rings but probably the most popular as they are the first thing that come up when you search on amazon. Mine are nearly 5 years old and still going strong - I love them. Whilst they have made some changes to the design, basically making them look more aesthetically pleasing with a nicer finish, they are essentially the same great bargain. Only downside is the straps are not fantastic but hey you can always buy others later on.