Simple Machines has
notes regarding the changes. A number of the improvements are on the administrative side or behind-the-scenes, so yes by and large it should work like before. The major visible change would have been the Curve theme, but we are using our own custom theme. Therefore the upgrade won't change the look and feel for us significantly. To give the site a facelift, we would need to revamp the theme ourselves, or switch to a different SMF theme that is offered to the public.
The most likely issues you'd encounter would be incompatibilities between our theme and SMF. There were many spots where code changes were required due to differences in the various functions and options available for theme templates to interact with in the current version. I believe I caught most of these, but it is possible I overlooked a few in the process.
