It is a sprequel. It ties together the First Class and original Xmen films, using a storyline from the comics that involves the infraction between the past and future. Fairly ambitious undertaking, the cast should be enormous, and already includes Peter Dinklage.
Personally I'm looking forward to it. Liked the 'chemistry' between McAvoy and Fassbender in XM:FC (even if the kids characters were slightly...... off. Which I believe we have discussed on here before)
I'm curious about it. First Class suffered for it's development - a car crash of 2 scripts after they failed to convince anyone that X-men Origins: Magneto, or X-Men First Class would stand alone so mashed them together. The 'First Class' bit was rubbish, the stuff with Magneto and Charles Xavier worked brilliantly well (even with Fassbenders slow drift towards Magneto'Malley). And the original films were... alright. X2 is a genuinely good film, 3 is ropey and 1 is a tentative step into the modern comic film world.
So weaving both timelines together in such an ambitious plotline is intriguing. Could be brilliant, could be a car crash.