Having previously tried the Slime tyre liners which I was really disappointed with, these Slime tubes were a revelation! Yes, they add a little bit of weight to the tube, but not enough that you're really going to notice after a while, and they mean you never need to carry spare tubes. If you get a puncture, just pump it up and keep pumping until the slime seals the hole, then carry on.
I will say, the tubes can take a while to seal when they're cold - on winter rides, I've had to spend a while getting them to seal, but once they do, you can ride on without problems.
When one of my tubes did refuse to seal and meant a walk home, I was all set to give it a negative review, but when I took the tube out to replace it, I started pulling out thorns which I'd picked up on a section of canal lined with Blackthorn bushes. There were no less than FIFTEEN big thorns in the tube - it was a wonder it had sealed at all. Now I try to avoid that section of canal, I've had the same tube in for months, problem free - with normal tubes, I was having to swap or repair on a near weekly basis.