Not so difficult to assemble.  It took me and my teenager two hours together.  Construction seems really thought out and durable.  My kids love it!  Looks like it will last. 
For sizing, keep in mind that the springs take up one foot on each side, so the 8X14 really turns into 6X12 jumping space.  We were limited on space, so this is what we needed, but I would go with a bigger size if you have the room.  It is perfect for jumping and flipping, but probably not for gymnastics combos, such as roundoff backhandspring.  My kids are only doing handsprings, which is really fine.

They love it!