Its not just about eye candy. In the beginning I did spend countless hours on with changing the lockscreen with Typophone and HTC themes, sliders, icons, animated wallpaper, Barrel, FakeOperator etc.
However I now prefer the stock look and jailbreak my phone for the functional things such as:
SBSettings
SBAdblockToggle
BiteSMS
Springflash (hold down sleep button to quickly activate LED torch)
Lockinfo (emails and notifications on lockscreen)
PhotoAlbums+ (Allows you to create separate photo folders)
Liveclock
Weather icon
ScrollingBoard (Quickly access iPod controls with one quick swipe on the dock)
CallTell (announces who is calling and reads SMS texts aloud)
iFile (Complete access to file system)
My iPhone 4 running 4.2.1, super fast and smooth, and has never crashed.