No spoilers, only logistical information:
The lightsaber is high quality, and worth $200 alone (IMHO). A pin (matching type of light saber) and case is included.
The "show" part of the build is a great experience. Each "builder" may bring 2 guests.
There has not been an advance reservation system during the previews.
Some reports from CM previews have waits of 3 hours

I was a guest during a corporate preview, which was much less crowded than the CM previews -- no wait to purchase, 30 minutes to enter, 20 minute build (times are estimates, since smart watches along with phones were not allowed).