I assume you mean these:
It really depends on the kind of sign. Mostly the ones that look the worst are those that businesses have bought themselves and they have a limited number of letters, they get faded, stolen etc. and over the years they start to looks like crap.
There are companies that rent them and will either change the message for you or give you some lettering to do it. The advantage being that they have the inventory and volume to keep the letters looking decent.
As far as I know there are 3 types of these signs:
- One has slotted lines and the letters/images slide into them and are easily changeable.
-Two is like a mini portable billboard, the same size but the whole sign is printed and the side panels need to be replaced to change.
-Three is a flat metal surface that you place magnetic letters/images on and are easily changeable.
I have relatives who used to be in this business and I have a preference, so if you're actually looking to get a sign for something I can give you some more info.