To his full credit, Keenum is the all-time NCAA leader in passing yards, touchdowns and completions. However, he was not drafted in 2012, signing with the Texans as an undrafted free agent and spending that season on their practice squad. Arm strength was a major issue - at the 2012 Combine Keenum didn't look capable of throwing any sideline route or even a deep dig - as was his size, as he probably goes 5'11". He also has a torn ACL in his past medical record. (Of course, that's one less than Sam Bradford.)
Keenum closed out last season for the Texans after Matt Schaub imploded, though, and he showed some positive traits. He's good with play-action. He buys time well with his feet. He plays without fear and trusts his receivers. He isn't afraid to go deep and has surprising success with it. He threw 9 TDs (unfortunately, it took 8 games) and 6 INTs. On the negativc side, he fumbled six times (losing two), still doesn't have a lot of experience playing directly under center, throws off his back foot and is still too easy to read. He completed just 54% of his passes and has a career record of 0-8.
Keenum sounds basically like Austin Davis, except with half a season of starting experience, so I'm not sure they're gaining a whole lot. My choice probably would have been Mike Kafka, but that was before Tampa picked him back up for their practice squad. Keenum's certainly got an experience edge over Garrett Gilbert, though, who's now on the Rams' practice squad, and it's wise of the Rams to have a third QB on the main roster. They've at least got a little professional starting experience behind Shaun Hill now.
The Rams will have to release a player to add Keenum to their roster but haven't announced who yet.