I use it to devastating effect in the early game - it's extremely powerful. Wolves and black widows are best off the start - stalkers will kill you, baby bears are too weak. I do think it's uses spell hit though - so if you're not a mage, it's going to miss fairly often. I'll often leave spider webs in my territory - when they respawn, I tame the new spider.
You do have to get the timing down - if it's just your sov in the army, you need to wait to cast it until they move one-turns movement away from you. Then you take 1-2 hits, cast, win. If you have another unit in the army, you can either let that unit tank the beast or move your second unit right behind your sov and the beast will often move next to your sov. Settlers make great tanks - every creature wants to eat them and it doesn't matter if they lose nearly all their hp, they can still found a city or an outpost.
Other creatures - all spiders typically work well, but don't try to tame ones way up-level from you, you'll miss. Ophidians (sp?) almost always resist. Baby bears die in like 1 hit, so they're kind of worthless. Other bears, nearly on level, are great. Najas are good. Stalkers are awesome (4 move), but them moving first makes them hard to get early in the game and hard not to kill with counterattack later. Umberdroths are awesome, but I haven't seen one in a while.
I just started a game with a mage / beastmaster sovereign, By turn 40-ish, I have 3 black widows running with my sov, 2 sets of 3 wolves with 2 other champions, and a single wolf scouting (4 move scouts FTW).
Edit: the game says you gain xp from the win, but it doesn't actually add to your xp total.
