I`ve used it during my euro trip last year.
its still in the front, and I need to re-apply it to the back tyre as its new.
There are pros + cons for using this product, as it stays liquid, just take it easy for the 1st few miles if the bikes been stood for some time as the sealant can settle at the bottom of the tyre. This allows it to re-distribute itself.
If I was riding around local, or the Uk only, I doubt I`d bother using it, but it adds a little "piece of mind" value to touring trips, and sure beats sitting at the side of the road for several hours waiting to be picked up by the tow truck in mainland europe, should you have the misfortune of picking up a punture.
By all means use it, but get into the habit of periodically checking your tyres for damage, as you don`t always get a tell tale green spot appearing, where the sealant has sealed a puncture. I would only use it on a new tyre, as the cost of applying it to a part worn tyre doesn`t make it finacially viable IMO
Edit: IF you do go ahead and put ultraseal into your tyres.
An important point to remember is, when you have to replace your tyres, DO tell the tyre fitter there is ultraseal inside the tyre, if you don`t or forget, you WILL be the most hate guy in the tyre store. tyre fitters hate the stuff, because it get all over them. But if you tell em, at least they will be forwarned & act accordingly