Apparently on Haruun Kal he has raw power that rivals Yoda's and Anakin's, but off-planet his power in the force is thought to be nothing compared to someone like Vader.
NewGuy01 summed it up well. Kar is very powerful but unlike most Force-users his power is very much dependent upon his symbiotic connection to the jungles of his home world.
On Haruun Kal he defeated Mace Windu in a Force-aided unarmed fight. Mace admitted that he didn't throw the fight, it was a legit defeat as Kar would have sensed if he was holding back (Mace was counting on the fact that Kar thinks like a pack animal. Once he'd established his authority he was content to not kill Mace).
In addition to power Kar is capable of great skill and control in certain aspects of his abilities, especially animal and plant manipulation. For example, he was able to manipulate the minds of parasitic larvae to get them to leave their hosts and could heal people of fungal infection by focusing his power to burn it out spore by spore.
In terms of fighting skill he's mostly a brawler, albeit an experienced one who's taught himself to fight effectively. In his fight with Mace it was clear that Mace was more skilled and landed more hits but Kar's connection to the jungle healed him too fast for Mace to wear him down.
In short, on his home gound he's on par with Yoda and Anakin but is very specialised in the nature of his abilities.
Kar Vastor also returned, many decades after Shatterpoint, in the novel Millennium Falcon(I think that's the correct title) to have a final showdown with Luke Skywalker. I don't recall anything impressive of Kar in that novel, though his fights with Mace in Shatterpoint were great.
__________________ "You must begin by gaining power over yourself; then another; then a group, an order, a world, a species, a group of species... finally, the galaxy itself."
- Darth Plagueis
"She may be small but she packs a punch and when shes in this mood...she goes for my danglers" - Oghren, Dragon Age
"The Daemon lied with every breath. It could not help itself but to deceive and dismay, to riddle and ruin. The more we conversed, the closer I drew to one singularly ineluctable fact: I would gain no wisdom here."
Oops, yeah. My bad. Those came out around the same time and I got them mixed up.
__________________ "You must begin by gaining power over yourself; then another; then a group, an order, a world, a species, a group of species... finally, the galaxy itself."
- Darth Plagueis
"She may be small but she packs a punch and when shes in this mood...she goes for my danglers" - Oghren, Dragon Age