I bought this on the strength of the prior two Riddick movies, and the first thing they do is abandon the Necromonger storyline, drop Riddick on a planet, and then put him up against the planet, some local fauna, and a (predictably) over-matched, underwhelming pair of merc teams with the predictably usual plot developments.
Consider, for instance, that this is a character survived Crematoria, and then took on - and beat - the leader of the Necromongers, and now we're looking at Riddick vs some merc teams? It doesn't add up. Sadly, the backstory is that Universal decided to shelve the Riddick character after "The Chronicles of Riddick" didn't do well, so the the original creatives got the rights back and produced Riddick, which means the follow-up stories and movies will be vastly scaled back.
On the plus side Riddick develops an interesting master / pet relationship with a local animal, one of the merc teams had a redeeming reason for being there, there's some unexpected plot twists, and Katie Sackhoff does a good job reprising her hard-scrabble characters along the lines of BattleStar Galactica and Longmire.
Overall I give this three stars as a "meh" movie - it's worthwhile if you can get it for around $10 and have an afternoon to spare or something else to do while it's playing.