It does shrink a bit with the heating on and expand in the summer (as the moisture content increases). But it's only the odd board that you would notice, and the increase in gap I've had is a millimetre or so. I've it in now (beech) for three winters and have no actual problems with it (shrinkage, curl, separation etc).
Some things to watch out for:
- my contractor said it should be stored in the house with the heating on for six-eight weeks with the heating on to acclimatise it
- the floors need to be really level
- make sure there is no chance of damp coming in under any of the door frames