I've been running the Vittoria XLs in a variety of conditions this season. Now, I'm no expert at this game, but here's my thoughts:
On hard, grassy ground, they're excellent - amazing side grip on off-camber stuff and very little drag. No rumbling when used on road, due to the tread profile down the middle of the tyre.
When it gets muddy, they're disappointing, because they just clog up easily and slip & slide all over the place. Side grip goes and forward traction too. Hard work to clear mud from them, even on faster dry concrete road sections.