I installed my own last year, had them dropped off on site. There are various
thickness available, availabliity may vary by area. The Home Depot 10' gutters
are .019" thick. The common seamless gutters are .025" and came in various
colors. The 'commercial' version is .032" thick and came only in white. I have
large trees that drop branches, so I went with the .032".

I assume that where the gutter turns a shallow angle (e.g. 22.5
degrees for a bay), a seam would be required anyway. Or is there some
way to extrude the angle, too?

It needs a joint. There are two types. mitered and mumble. One type is the
standard, 6" long, pre-formed corners you can buy anywhere, the mitered joint
is a small strip that goes inside and your miter cut (plus the rivets or
screws) is the only thing showing. I only needed 90 degree corners.


On a long straight run with downspouts at either end, the middle
should be high with slope down towards each downspout. Does this
require a seam in the middle, or is the extruded gutter flexible
enough to accomodate the change in slope without a seam?

Hmm, dunno. My longest run was 36', and I designed it for one outlet which
works fine. They are pretty stiff when installed, they certainly wouldn't drop
at each end very much without a relief cut or a kink in the center, I would
think.

--
Dennis