I've been eyeing SF III for a long time now. Big FE, Advance Wars and Tactics Ogre fan.
How is the SRPG gameplay of SF III? Does it have some depth and challenge to it?
Oh absolutely! Can't recommend it enough! An absolute gem.
The map design in that trilogy in particular is just fantastic - once you're past the intro, almost all the maps include at least some kind of complication.
Time limits, restrictive terrain, unbeatable enemies you need to flee from, seemingly endless reinforcements, a third faction, the list goes on.
It also has plenty of (at times hidden) side objectives and optional approaches sprinkled throughout that unlock different characters, rewards or story events - some of those even have consequences in the later games of the trilogy, should you import your save file from game to game.
It can be on the challenging side of things, assuming you actually want to see all there is to reasonably see, some optional objectives can really kick your ass. Simply beating the maps is usually manageable though - it's easier than Advance Wars 2 for example, I'd say.
Seeing as you're a fan of FE and Tactics Ogre, I also just find the overall structure of the Shining Force games interesting.
This series places a greater emphasis on the (J)RPG side of the T/S RPG equation than you might expect, with explorable story environments between maps taking up a large part of the experience.
Should you check it out though, absolutely go for the Japanese voices for the first scenario - the English voice acting is infamously awful and while it can be funny for a bit, it really is distracting.
(Sorry for the wall of text - I love these games and I tend to jump at the opportunity to gush over them.)
Digital Devil Saga 
