I'm a big fan of this toy. It's a great combination of cold therapy with the obviously use of chewing for teething babies. I've used the Munchkin Ring on two of my children now with great results. I tend to take it away as soon as it gets warm though as they aren't terribly fond of it by that point. It typically lasts about 20-30 minutes. It takes a little bit to freeze it up again too, and obviously freezers aren't portable so be sure you're not relying solely on this for a big day out! Your child will be kicking and screaming if you do.
I alternate between a lot of different chew toys. Some super hard, some a little more flexible. It really depends on the baby which one is most effective. I always carry a rattle and have grown to wear a teething necklace as well; Pendant Baby Teething Necklace - Non-Toxic Jewelry for Mom, Safe for