I have heard of them but have never specified them. They do have an ESR report http://www.icc-es.org/Reports/pdf_files/load_file....

I seen them used before but have no results either good or bad. I wouldn't worry too much about using them.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com

We are currently using them on one of our projects to couple column reinforcement. They work well when installed properly. You'd be surprised how many contractors haven't used them before and install them incorrectly (as crazy as it sounds installation is not necessarily fool proof). My only word of caution is that the ESR report that WannabeSE linked to indicates their use is for deformed steel reinforcing bars. Other applications may go beyond the scope of their testing.

I saw them used on a ring foundation

In this part of the world a similar type exists via a few suppliers, one common property they all share is that they can yield the bar, but they do not possess sufficient strength to fracture the bar (to reach the ultimate tensile capacity). This means they are generally unsuitable for any application where ductility is required

There are two specs of splices, one that will take full specified tensile (Type 2) and one that will go to 125% of yield (Type 1). The Barsplice products are generally very good products, along with Erico, Dayton Superior, and a number of others. You should be ok as long as they comply with the relevant ASTM a1034 for testing splices.

Probably not a problem in a tension tie rod application, but be aware that the splices take up a fair amount of room, and may cause dislocation of bars or loss of cover.