Very effective as a battle mount/travelling companion. I mostly play on single player so an effective companion is vital. I tamed my Kopiona when she was level 32...she's at 62 now with 200% melee damage, over 800 health and quite a few surprising kills notched into her tail.
Some noteworthy accomplishments (without my assistance) are:
-1 Carno killed (lvl 30)
-1 Spino (lvls 3)
-1 pack of 4 Raptors (lvls 36, 24, 18, and 50)
-Survived a Rex attack (lvl 24) after I had died with minimal health loss.
Coupled with its torpor inducing attacks, it's also great as a taming mount.
Scorpions make a great early seafaring mount if you hold the jump key ( space bar by default) a scorpion will float above the water allowing you to get past most Puranas without being attacked, this can be extremely useful for crossing dangerous rivers or even oceans.