It's been pouring rain since they arrived, so I haven't been able to set them around my cantilever umbrella yet...but they look sturdy and once I have filled with sand, I'm sure they will do the trick in hold the umbrella in place - even on windy days.

Update:  the connectors do not fit in the holes, so I placed large planters on each of the pieces to help hold down the umbrella.

Ok...it's been a few days.  None of the connectors, that are meant to connect one piece to another, fit.  They seem to be off about 1/2 of an inch.  I've tried everything, but can't get them in, so I'm trying them right now without the connectors.  If I need to, I'll have 4 made to fit.