We have used direct bury Cat 5 and the cable worked fine.  We did put it in PVC conduit, but the conduit was cheap.  But after two lightning strikes I would not bury Cat 5 cable again even if it was the only option.  The first lightning strike got a switch and the NICs in two computers.  That was not very expensive.  The second lightning strike got about $10,000 worth of computer equipment.  Every device attached to the network was damaged and most was a total loss.  We salvaged hard drives and managed to save data, but the hardware was toast. 

After that experience, we replaced the Cat 5 with fibre optics.  It is not cheap and you can't put the ends on yourself, but it was worth the expense in the long run.  I would use wireless or if that is not fast enough, go the fibre optic route.  You will regret buried Cat 5 at some point in time.

I still have a partial roll of direct bury Cat 5 in the garage, but it will never see dirt here.