Texas has requirements for the sign.
It must have block letters no smaller than 1 inch, must refer to the CHL regulation # 30.06, must be posted at every entrance to a building, etc.
That being said I will not enter a building that has a sign posted prohibiting possession of a firearm, if the sign is legal or not. I avoid places with the pistol in a red circle with a line through it, I avoid places with the handwritten "no guns" signs in the window, and I treat them all the same as the sign that is to the letter of the law.
The way I see it I have everything to lose and nothing to gain. I might prevail in court, but my CHL would be suspended for the entire time and it would cost me mucho $$$.
If the owner or tenant has made a resonable effort to prevent the legal possession of a handgun on his property, I oblige him and make a reasonable effort to prevent him from any of my money. There are plenty of places I visit/shop/do business with that have "legal guns welcome" or "possession of an un-licensed firearm is prohibited etc." I just spend my money there.