Another Hero owner with an Optimus S...I have to say, you shouldn't hesitate to switch over as the Optimus is a definite upgrade. So far it has been a VERY solid phone, no lag/force closes of any sort. Very responsive and boots up so much faster.
My Hero was rooted with ZenHeroFX 2.0 ROM and I thought it was solid then. I was up for an upgrade and considered the Optimus as it had a similar form-factor as the Hero and it came with Froyo, a bit more memory and a faster processor. I was not disappointed at all and the phone is so smooth. I'm very happy with this phone.
However, I'm going to be a bit nit-picky and say that the camera lense and picture quality are not as good as the Hero. Comparing them side by side, the Hero takes better pictures and video. Also, there is no LED indicator of any sort on the Optimus...so no more knowing whether the phone was charged, received a text/email, missed a call or anything of that sort. Still trying to be creative on how to workaround this. I also do miss some of the HTC widgets from the Hero as the Optimus was lacking some of my favorites right out of the box such as the full calendar, people widget, clock w/weather, stocks, a better music player widget and etc. I did find alternatives in the market but not quite the same. But I'm willing to sacrifice a bit on the widget-end for the stability, speed and responsiveness that the Optimus brings.
Bottom-line: Despite the shortcomings (IMHO), you can't go wrong with this phone. LG did a good job and I'm impressed with the build quality. My Hero was falling apart at the keys...I still love my Hero, it's just getting old and worn out