I'd recommend starting with Intervention, then moving on to the Galactic Milieu trilogy (Jack the Bodiless, Diamond Mask and Magnificat) before reading the Saga of the Pliocene Exile. Intervention is, IMHO, the finest single book Julian May has ever written.
I would agree with that, except for one tiny detail:
They weren't written to be read in that order.
They certainly weren't written in that order, but reading them in that order (which I did last fall, one right after the other) didn't introduce significant spoilers, apart from the one about Marc Remillard.
Seriously, try reading them in that order and see what I mean.
I always think it's a good idea to start a series by reading the strongest book. Personally, I find the last 2 Pliocene books to be overly padded to the point of tedium -- I've been "lingering" over the last half of
The Adversary for 8 months or so, trying to force myself to finish it. If I'd started with the Pliocene quartet, I might never have read the glorious
Intervention.