Viper lines and up are great.
The Diamondbacks appear to be a good value,and compete with Bushnell 3200, Nikon Prostaff/Buckmasters, Burris Fullfield II,etc.
The lowest Crossfire line can be nice enough, but aren't as well put together as the higher lines.
They have a lifetime transferable warranty, and my Viper at least has good glass and tracks very well.