I have never used Essie and only used the odd Orly polish so can't really comment on either.
I stock OPI as it ticked all the right boxes for me and I also like the fact they have a rep so I can actually try products in the salon before commiting myself to a purchase.
When comparing OPI & Jessica its tough for me as I prefer OPI polish but I love the Jessica treatment base coats & the products, they have a very luxurious feel imo. However I have to agree with some of the comments about the polish going gloopy and the application.
I have attended the Jessica training and Martin is right there is a specific application which ensure the polish lasts longer, my problem with this is that the clients either can't do it themselves or couldn't be bothered anyway so the polish doesn't last as long as other brands when applied at home.
With regards to the training I feel it is a must regardless of the product range you take on and the reason for that is every range has a different routine for their treatments.
For example the CND Spa Pedicure is probably the best ever pedicure routine ive followed before. I still use mostly CND for pedicures and I get so many comments about it being the best pedicure my clients have ever had because its so comprehensive, now imagine they went to another salon down the road which hadn't done the training and offered the same pedicure but the routine was just a simple one learnt at college, I would feel a bit short changed if it were me and it has happened several times when ive been for treatments elsewhere !