Downside at the minute and it is a biggy is that you lose your warranty for unlocking the bootloader. However with the kernel source released the devs and community at large are looking to achieve a non HTC involved security off exploit (S-OFF) so as to achieve root and more without losing warranty and to allow the phone to be put back to stock before you make a warranty claim.
This approach for me is highly beneficial as the device is less than 12 months old and its cost a small fortune I don't want to be throwing a valuable warranty away. With 1.29 and the Apex Launcher I'm happy atm. El Presidente and D-U-R-X will vouch my flashaholic tendencies so trust me when I say I want root bad. However for me it really isn't worth the risk of having issues getting hardware related issues fixed because of using htcdev.
HTC Bootloader and Warranty an on-line conversation - xda-developers
DO NOT UNLOCK your HTC One X through Htcdev.com - HTC will never repair your phone - xda-developers