The only bad thing that I have heard about the saliva monitors is that it can take a few months to really get the hang off it.
I guess reading "partial ferning" from "ferning" can get a little shaky when you don't know exactly what you are doing. But it will just take more effort then the opk's. However, you won't have to worry about running out of strips before you catch your surge.
I say they are worth a shot. I was thinking about getting one when I start ovualting.
Keep us posted and let us know if you decide to get one.