It's not that great. The best barb ring in the world would have exactly the following stats:

+100 Vit and + 100 Str (first prefix)
+80 Resist All (second prefix)
+80 Physical Resistance (third prefix)
+12% Life (fourth prefix)
+100 Str or + 100 Vit (suffix)

Prefixes and suffix don't show up on rares, but the stats are calculated in the same way as blues. The only difference is that rares aren't limited to 1 prefix and 1 suffix.

Gem slot is always a wasteful prefix. Since instead of a gem slot it's possible that the prefix could have been 80 resistance or 100 Vit/Str or 12% Life. Gem slot is especially wasteful on an amulet, which offers nearly double the stats for most affixes (i.e. 200 Str instead of 100 Str).
Edited by GhoXen#1998 on 5/25/2012 6:18 PM PDT