I've noticed a minor issue with the plugin, not quite sure if it's a bug or a feature.
When an input error message is displayed at the top of the form, the message is wrapped not just in the error message div, as one might expect, but also in the required field div. As far as I can tell, this isn't optional, it appears to be hardcoded in the plugin's si-contact-form-display.php file.
For most people, this probably doesn't cause any issues, as the error message CSS can override whatever's in the required field CSS, but for anyone specifying relative font sizes in both, well, it sort of implodes. ;)
I was able to "fix" the issue by hacking the form display file, but that's obviously not an ideal solution, as it will have to be re-fixed whenever there's an update.
Would love to see a real fix for this issue if it's a bug, or the additional div made optional, if it's intended as a feature.