Most of the cheap ones have dots that cannot be seen in bright noon summer sunlight against most backgrounds.
The ones that do have bright dots, (like the design above) are usually fragile.
I'd like to check out the brightness of an Ultradot sometime. For $125 a lot of bullseye guys swear by them and say dot is visable and can handle stiff recoil. They have a lifetime no BS warranty, so that's nice to know in case it does break. Only thing that sucks about them is battery life. But that is true of everything except EOtech and Aimpoint.
Burris has the speed dot for a little under $200. That is suppose to be nice and bright, and reasonably tough. I think for low $200's you can get a Holosight by Bushnell (made by EOtech, but without the armoring) Also the Tac-point which was kinda-sorta torture tested here on AR15.com a while back. It is an Aimpoint clone that many feel does everything you need it to do for most uses...except combat.