I have heard lots and lots of good things about Tilley hats, and I see people with them on the trails all the time. When I have looked at them in REI, I have been impressed with the construction.
I would like to buy one, but I have never yet at REI seen the models I want in the size I need. Depending on the cut of the hat, I take either a 7 and 3/4 or 7 and 7/8. Unfortunately, I have not yet been able to find these sizes to try them on. Because of the cost of a Tilley, I really think I should try on one before spending the money.
I have the same problem buying motorcycle helmets. With my 63.5 cm round head, I usually take an XXL or XXXL, depending on the helmet brand. Virtually never do motorcycle shops stock helmets in those sizes to try on.
As a possible alternative, Outdoor Research and Columbia also make some nice hats.
PS: not to mention that for a lot of hat or motorcycle helmet manufacturers, XL is the largest size they make, which further limits the selection for those of us with large heads.