Sieges are huge fun, at least the escalades that tend to be played at 28mm are, you just need to get the rules right and catch the right group, same as any other game variant (Like skirmish, cityfight, Planetstrike, whatever). I forget which version of Warhammer siege it was (5th?) with the release of the plastic fortress and siege tower and all the metal gear that provided loads of great games, it works quite well in the LOTR system too.
But yeah I actually think fantasy systems work even better given the general disparity between troops and extra factors like flying and magic, though those things do need toning down a bit, like entirely flying armies or Lord Kroak tend to render the fortifications pointless