If your PT session has lots of clips that don't start at the beginning then I think OMF becomes a valid solution. It's just a tool that you would use if you've tracked things in PT and are beginning to mix in Sonar. Just make sure you note the tempo in PT as Sonar will ask what it is when opening the OMF file. I've done this and everything lines up perfectly in Sonar. In fact, I was excited how easy it was to move from one Program to the other.
Now if you've done a full mix in PT and want to move that into Sonar... that's a bit tricky. It can be done if each plugin you've used in PT is available as a VST and you're able to use the plugin itself to save the preset. You'll have to match the panning law to what was used in your PT session. Any VSTi's will have to be printed beforehand... and all automation, panning and levels will have to be done by hand... if you have a bunch of automation then it starts getting to the point of asking why you would switch programs? If you're mixing someone else's project then I don't see why you wouldn't just start from scratch anyways...
The benefit of using OMF over simply importing Broadcast waves is that OMF will remember the track layout whereas importing Broadcast waves will put each clip on a different track. For example, if you have a guitar solo during the intro and then again at the bridge and they are two separate clips - Importing waves will put them on separate tracks while OMF will put them on the same track. This isn't an issue if you only have single clips per track... but if there are a bunch of overdubs in Pro Tools, then I think OMF is definitely the way to go.