these are the only way to go. i have had the same slime filled tubes on my downhill MTB all year. when they go flat, pull out the sticker and refill the tube with air. it should seal just fine. the only slight problem is that they are heavier than air tubes. this makes a difference in something as light as a mountain bike, but doesnt make much difference in a motorized dirtbike. they arent as strong, but they are wayyyyy lighter than a solid rubber tube.
these are commonly sold at your local bike store, here in california they are $14 per tube for a 26" bike. cheaper as the size goes down. with the couple dollars more that you spend, you wont have to replace the tube every time you get a flat. it gives you awesome piece of mind when out on a dirt trail. i am more worried about someting going wrong mechanically than i am about my tires. this pretty much takes all of the fear of getting too far away from my truck right out. i am finding myself going longer distances and worrying less about the bike, and staying more focused on the trail.
it is somewhat like fix-a-flat, but they dont fill the entire tube with the slime, and it does not expand to inflate your tire like fix a flat does. but yes, its generally the same concept.
