I've been using one for close to 20 years. I have an pre-luber mounted to the fender well in my van. I flip a toggle switch and watch the oil pressure gauge, when it's in the normal range I start the engine up. I also use it to cool the engine after a long trip in hot weather, flip the switch and pump oil through the engine after shutting it off.
The main reason I bought it was this vehicle sat for weeks on end, and it was nice to get oil flowing through the engine after it sat for a long time prior to starting it.
As a side note, during winter time synthetic oil 5w30, pumped through the pre-luber will build pressure and start flowing long before 5W30 dino will.