I used WidgetLocker on both my myTouch3G and my Vibrant. WL worked great on the Vibrant, but had problems doing more than just "unlock" and "set to vibrate" on the MT3G. I used WidgetLocker for the widgets - I had clock, battery status and a music player widget.
I switched to Agile briefly on my myTouch4G Slide because it didn't come with the full-blown Sense3 lock screen, but I found Agile to be very... wonky. I'm now using the stock SenseLite3 lock screen on the MT4GSlide.