I have both Solo and Solo II. I like the faster warmup time for the Solo II, but I prefer the way the original Solo starts up with just a twin button push and hold; much faster than the Solo II. I find myself starting that countdown to the smiley face several times, probably because I have them both. It is a cool feature but it annoys me. The Solo II does complete the warmup much faster. But even though there have been many excellent design improvements I find that the Solo delivers the vapor just as well as the new model. During the winter the Solo can also be used as a hand warmer(great for thawing half frozen fingers), something that the Solo II just can't do. I feel the Solo II is safer to use in a bed because of this; Solo II was hot enough to melt a rayon pillow case one night. I really like both of them. Well worth the money. They seem to last forever with an occasional battery change.