M Xp Certainly is the easiest xp to get for most guilds but it is very unbalanced.
As for spells giving xp if they don't work I think it should be looked at on a spell-by-spell basis.
I think if a CL has a particular bind on (let's say bind greater demon) and they want to be fully bound with that, they should be able to get xp for the spell cast simply because it is one of their bigger xp spells and it has a duration that makes cycling it unreasonable (something in the range of 2 hours).
A spell like res however I don't think should give xp on a fail. I understand that res is a higher level spell and very good xp and not easy (or possible realistically) to cycle however the spell itself is extremely good and rewarding. People should train it to help out others not to level quicker.
I view res like I view TA and TW. If trues gave -10k xp every time somebody casted them people would still use them so the crappy xp a chanter gets from the trues is a non-issue. Res is rewarding in it's own way, and is a very powerful spell.
If you say plz because it is shorter than please, then I will say no because it is shorter then yes.