Are they worth spending sP on?

If you mean worth it in terms of loot, no. But then, if your a serious battler its not about that, its about the challenge.

Very few challengers are worth it in terms of loot, but considering how easy sP is to come by, I'd easily say MC challengers are wotth it.

OK. Thanks.

I was thinking worth it for gaining XP as opposed to buying stat boosters.

For some MC challengers it's worth it for the experience. It's also worth it if you have multiple battle pets that can boost from experience.

Thanks. I've bought the first 3 and done 10 battles each with my main battle pet