Once you root the phone, the first thing you want to do before downloading and installing any ROM, perform a Nandroid backup (home+power on for restore menu, when the phone is off of course.) This will be a backup of your original, unmodified Hero rom.
Once you've backed it up, hook your phone back up to your computer and make a backup of the Nandroid folder just in case you accidentally delete it off your phones SD Card.
Then download the Fresh 1.1 rom, boot into the restore console thingy (Home+Power on) and install the new rom from the menu (it needs to be renamed to update.zip.)
Once it's done installing, you're up and running with Fresh 1.1 rom! At any time, you can download a new rom either from a PC or the computer, rename it to update.zip and reflash the new rom.
Currently, the 2.1 rom that is available from Fresh is in Beta, some stuff, like the camera, don't work. Also, since it's a developer ROM, you won't have access to secured purchasable apps. ie, a large amount of apps are not available.
As for a complete 2.1 ROM with or without sense is not currently available. There is a developer rom of 1.6 without sense that I think works, but i haven't tried it, and I don't think it has all of the app store apps.
Changing roms is easy, as is backing up a rom that you like. Nandroid restore will restore the latest backup. It stores them by date, so if you backup the original, install Fresh 1.1, perform another backup, then download and check out the Fresh 2.1 rom and decide "Meh, I wanna go back to Fresh 1.1" just run the restore and you're back to your Fresh 1.1!
If you want to go back to the original hero rom, then remove the Fresh Nandroid backup (again, it'll be the most recently dated folder inside the Nandroid folder) and perform a restore - done, you're back to your stock build, though still rooted.
To completely unroot the phone, check out the CDMA XDA forums because I haven't done it, nor do I plan on going back to un-rooted.
Just make sure that you do NOT install anything found outside of the XDA CDMA forums. You'll see other roms in the hero forums on XDA, but unless it's in the CDMA forum it's not going to work and you'll have to either reflash or do a complete wipe (all data on the phone) and a restore of a Nandroid backup.
I've not heard of anyone bricking their phone. I just suggest doing it with a full battery, the battery cover securely installed (don't want that battery popping out mid-flash!,) and patience. It takes a few minutes, don't get worried it didn't work and go and unplug the battery or try to turn the phone off. If you do, you might brick it, but I can't be sure even then that it won't be recoverable.
Once the phone is rooted with the recovery ROM installed, it's really hard to brick the phone.
Edit:
Also, if you experience issues such as force closes or apps not working etc, it's usually caused from some kind of corrupt data or something didn't get copied over correctly.
For the best build, I suggest downloading the Fresh 1.1 rom, doing a wipe from the recovery, and installing the 1.1 ROM. You'll have to re-setup your phone (ie, log into a google account and re-download all your apps and adjust the settings etc.) but you'll be less likely to run into little quirks and issues.
An un-rooted phone is seldomly 100% problem free. I think we all experience a few force closes every now and then (for instance, Google VoiceRecorder is crashing non-stop for me, but I haven't done a clean wipe/install yet.)
It's worth it in my opinion to have the faster (it's a lot faster) phone, wifi tether (it really is awesome having internet anywhere for my laptop and friends laptops) and the other uses and customizability you'll have access to.
There are some in the XDA forums who use the tether for their home internet connection. It's really quite fast, faster than the phone lets on (the phones CPU is a limiting factor for loading web pages.) One even said he hit 48 GIGS downloaded in a single month and hadn't heard anything from Sprint, but I wouldn't suggest doing that much!
Hope this helps!