It's easy to get used to it - it fits perfectly, so it shouldn't slip off and you shouldn't need to keep sucking it back on.
Yes, I think it does help, but you won't really know until you try.
One scary thing about a splint, my dentist (private) charges £650 for a splint [shock} - about 10x his charge for a sport's mouthguard (they look the same).
For daytime clenching, I find that chewing gum is effective.