I agree with the comments of Stephen F.

I would only add that if you are on the budget, then you do not necessarily
need to buy the crossbars and systems from Thule or Yakima.  Having said
that, Thule makes racks and boxes that will fit directly to the factory
bars - saves $$$ by not getting ancillary equipment.  These are easier to
mount and remove, but your expansion options might be limited.  You can also
buy a generic brand roof rack in one of the national automotive stores, and
then make your own adapters for the factory bar - I did it with a bicycle
rack which I got for my Focus (SportRack expendable bike rack, $45 Cdn).
The plus side of a Thule or Yakima systems is that those are usually
transferable if you got another vehicle, are of good quality, and there are
variety of accessories you can attach.  You can also buy fabric cargo boxes,
but I do not recommend those.

As for carrying stuff on roof, I found it very convenient.  It is quite
popular way of hauling cargo in Europe - I've seen many wagons, including at
least two Focuses, with roof boxes.  Personally, I think it expands on the
concept of a"sport" wagon.  Expect a bit of a penalty in fuel consumption,
but a bonus in capacity.  As long as you securely and evenly mount the roof
cargo (in cargo box, crate, or in a ski or bike rack etc. - see Thule site
for ideas), sometimes with help of tiedows or bungee cords, and within the
combined weight capacity, then you should not have problems.  Also, make
sure that whatever rack you get, it is lockable for security and some extra
peace of mind.  One downside is that with the higher Focus Wagon roofline,
it may be difficult for some to load the roof.

Hope this helps -

Cplgi
'00 Focus SE Wagon (and loving it! - still)