InRoads V8i vs. InRoads XM (8.9)

Sharing files back an forth may not be an issue but you do need to be aware of differences in the preference file (XIN). They are NOT the same. Take note of Tools>Oprions>>Geometry.  V8i has a new option to allow the horizontal and vertical alignments to unlocked (station values) from one another. I had some issues with that. There are other enhancements in V8i that makes it nessessarry to create a separate XIN file for each version.

Is it worth the hassle to upgrade from XM to V8i?

Read the What's new for V8i'. If you see a new widget you can't live without then yes it is worth the effort to upgrade to the current release of InRoads V8i.

With that said I would suggest you read the Whats new for the next release of V8i. The upgrades to MicroStation looks worthwhile all by themselvs. The enhancements to InRoads will be fantastic or fatal depending on how my users react to the changes.  I hear the next version should be out the end of the year. I hope to upgrade to it in March(?). The enhancements will give the designer a great deal more power. Fun, fun!!

Does anyone know if the templet libraries (.itl files) will be fully compatible between XM and V8i?

I've been bouncing back and forth between versions and have had no issues. I'm not saving any new preferences yet, but have never had an XIN file blow up after using it in a newer version. I'd be willing to bet that it simply ignores that which it cannot or does not use. The programming code to read and write xml files is pretty bulletproof. I've even editted XIN files in a text editor and had different closing tags than usual as a result but since either method was valid XML, InRoads cheerfully read and used the files without error.