Pros
Lots to like here: deep, hard to duplicate technology but wrapped into a business application that is easy to package and sell . Really good opportunities for career growth since you are literally building everything with the team; Great customer list for such an early-stage company (the names are kind of insane). Open, transparent culture - you will be involved in all key decisions if you want to be; Strong founding team that mixes deep engineering backgrounds (PHDs from CMU) with a very experienced SaaS CEO; really great group of people; should be a big market too, so lots to like thus far...
Kontras
Palo Alto location may be tough from some SF locations (not sure when/if they will move further north but i'm sure it has to happen); this isn't the place if you are into free massages and stuff, the focus is on building the company not perks; Also you have to learn quickly and teach yourself, everyone is running fast; Be prepared to work hard but it's not mind-numbing boring work either..