I'll take a crack at this, but Gerard is really our man on this one. The variable geometry (flippable) seatpost head on the Cervelo P3, Dual, P2K and Soloist models is a tool that affords a high degree of fit latitude, enabling the rider to induce a somewhat wider range of effective seat tube angles than a fixed head. It is not a cure-all and does not perfrom miracles. Good fit principles, especially weight distribution, must still be observed. In my opinion the Soloist with the seat post head rotated forward and the saddle slid all the forward on the rails is still a remarkably stable and well-handling bike- for some reason a lot better than just putting a Profile Fast Forward seatpost on, say, a Trek OCLV or whatever and cranking the saddle forward on that. That would really be a "nose heavy" unstable riding set-up. the Cervelo, again, for some reason- still rides pretty good. Much better than I thought. Having said that however, it is unreasonable to expect to be able to go through the entire range of fit at the seatpost without having to make changes to stem length. So, the seatpost is a big part of the package that gives the Cervelos versatility, but it isn't all of it. You still need to apply sound fit principles in addition to using the reversable seatpost. It is, however, an excellent tool.

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Tom Demerly
The Tri Shop.com

To add to what Tom says, with the Soloist you are only moving from a 74 to a max 76 degree effective seat angle using the flippable seat post. This is quite acceptable on a road bike, especially one with a compact frame geometry which seems IMO to lend itself a bit better to this than does standard road frame geometry. With a forward seat post this angle is closer to 78 degrees. This puts your weight further forward and you'll notice the instability in winds, cornering or downhills.

I don't own a Soloist but my road bike is a TCR-1 which has a similiar compact frame design. I can get the same geometry as the Soloist by using a M2 Racer seat shifter. The TCR feels stable to me at 75/76 degrees but when I ran it at 78 degrees with the forward seat post it never felt quite right except on a strait flat road with no wind. Two degrees of seat post shift doesn't sound like a lot, but it does seem to make a difference.

To add to this you can also get an approx 76 degree effective seat post angle on a road bike by using a forward seat post if you jam the seat back on it's rails. Dan put up a really nifty chart in this article that shows how by measuring the tip of your saddle in relation to your bottom braket using a plumbline you can estimate your approx. effective seatpost angle with fore/aft seat adjustment. http://www.slowtwitch.com/mainheadings/techctr/bikefit.html