I used to be highest tier in all three alliances, now that CO has left SkyTeam I have no status with SkyTeam (Has a BA Gold and multiple Star Golds). Historically I had been with KL, switched to CO when I moved to IAH.

Pros of SkyTeam:
-Good coverage from both my home airports- I need to transit in most cases, whether I fly Star or SkyTeam. So from a destinations point of view, SkyTeam is almost as good as Star (mainly thanks to AF/KL which goes everywhere).
- Fares are usually less in biz compared to other alliances ex-Europe.

Cons:
- No great carrier like SQ of CX. AF is the best of the lot says volumes about the kind of airlines in this alliance. I can bear AF (CDG transit is a nightmare), KL is ok, KE is reasonably good but they don't fly to too many places. I would avoid DL,AZ and SU unless I have no other options.
- No US lounge access on domestic itin. With a non-US Star Gold I can access US Star lounges
- Poor recognition across the network
- No intl F cabins (except AF).
- Poorer lounges compared to other alliances.
- Redemptions are the most difficult among the three airlines in premium cabins, ex-Europe as you have pretty much only two airlines to choose from for non-US itins.