Some simple guides from my perspective:
1) The best solution is a feed from the desk, ideally a separate mix for each band member. Most desks have more than 4 aux outputs used for monitors/IEM's. Latest digital solutions also mean you can control your own mix on your phone/Tablet. This is what my band does..This means everyone needs their own IEM solution. I've seen people trying to take a feed from their Kemper and sort of mix it with ambient sound but I think this is least successful. Definately avoid the 1 ear in and one out!
2) The wireless aspect generally is not what determines the quality but the headphones/ear buds used.
3) Good quality in ear buds are expensive!
4) I've never got a really good sound in IEMS despite good quality equipment
5) IEM's isolate you from the audience. Quite a few times I've taken them out to realise people were cheering after a song!
....BUT the benefits are huge. No backline needed, ear protection, being able to hear everyone makes your playing way better etc.