Note that eriksonlk has a x51
v. A x51v and x51 are different devices.
From my opinion, the ROMs (Operating Systems) released for the x51 are not mature. The new release by Lenny does not have a working CF. It does have a working SDHC.
Windows Mobile 6.1 in general offers multitudes of new "stuff". It is generally faster. The start bar and the bottom bar have gradient. It is more responsive than WM5. Also, it includes some extra utilities like an integrated task manager.
Despite WM6.1 sounding pretty good, WM6.1 ROMs for the x51 are supposedly not stable enough. They can easily crash and cause problems. So, the disadvantages outweigh the advantages.
If you insist on installing a WM6.1 ROM,
here is a link to look at. I can assist you in flashing (installing) that ROM.