In reply to bpmclimb:
I'll second everything 'Jmacquarrie' said, especially the comment about ditching the supplied felt.
The other advice I'd add is most manufactures have two grades, budget and quality. If you're after something solid, skip the budget stuff, you really do get what you pay for. Sometimes the more solid ones are sold as 'workshops' rather than 'sheds'.
If you're going to spend a decent amount on a shed don't forget about the price of a good base. Keeping it off the ground is a must (and not to close to fences) and allow plenty of room for air to circulate to allow the timbers to dry quickly.
Read the small print on any guarantee, some require a certain base construction and/or that the timber is retreated every year or it will be void.