ElectoralMap.net is currently forecasting a Republican gain in the Senate of 5 seats - not enough to take control. But there are three tossups in Colorado, Alaska, and Iowa that could go either way. If the Republicans pick up just one of those seats, they would have the 51 votes needed to take control.
So what are the Republicans' overall odds of picking up the Senate? According to the configurable election simulator it looks like a coin toss. Down from a high of 57%, due to events in Kansas, the forecast is now hovering at about 51%.
Note that the simulator is configurable -- if you don't agree with the polling in a particular state, just use the sliders to adjust the probabilities to your taste and press the "run simulation button." Your custom version can also be shared via provided URL.