I'm using ScanFont 5 with Snow Leopard successfully, but have not upgraded to lion because I value ScanFont more. After upgrading to Snow Leopard, there are some issues with ScanFont (hence the fear of upgrading to lion). They can be resolved or worked around with a little help. You may have some icon problems, but who cares. Most notably, for me, you loose the ability to open files by dragging them onto the ScanFont icon (you have to use the "open" command in scanfont). It's not a horrible glitch, just changed 1 step of my workflow. Also, expect frequent crashes if your merging and deleting cells quickly. Work slower, and you'll be fine. Finally, unless you install/use "rosetta" (fontlab sent instructions about this prior to the SN release/upgrade) SF will be very crash-prone (exporting, selecting the 'scale tool', etc).
As mentioned, most of this is minor and after getting used to minor workflow changes I'm still using (and loving) SF just fine. Wish I knew what would happen if I upgraded to Lion, but i'm not willing to take that risk until I know it can be done without major problems.