I've been using an EVO for 3 weeks now, and love it, dunno about the complaint about screen brightness, it's fine for me.
Biggest issue is battery life IMHO, and if you have a couple of chargers or an extra battery, it's a non-issue.
Sprint has an HTC slider coming out soon, the Knight, but I wouldn't expect to see anything else from them before the holidays.
The 4G really doesn't factor in to day to day use, if you tether, it's a big plus, and the Developers have a ton of custom roms out there, I have 8 different roms I can swap to, if you don't need the 4G, Cyanogen gives the best battery life I've found, but no 4G yet.
I could have bought anything, and switch carriers like I change my socks, I love my EVO and Sprint so far... I've been out of contract for years, and I bit on the EVO...