It depends on how wet it is. If it's a large storm, then the trails are slick as snot. Normal/easy stuff becomes tougher as it's hard to get traction. For example, one V-notch which is easy/med (at the most) when dry is pretty scary when it's muddy and slick.
We went a year or two ago when it was poring, made things a little more tricky!!! You learn a lot about how well your tires clear mud!!! I enjoy Hollister Hills, but mostly for the camping/family feel.
They let both 4x4s and motor cross/trail bikes on the trails in Hollister. The trails are marked according to difficulty (just like ski runs) diamond, double diamond, etc. It has been my understanding that a single 4x4 can not enter and go trail riding without a buddy in another 4x4. Guess this is so you can pull each other out of a ditch should the need arise. Camping is allowed there also. If I am not mistaken there are hundreds of miles of trails in Hollister. Used to go when I had access to a trail bike. It is a blast.
Father is boss as everyone knows,
But what Mother says always goes.