Ive always cleaned and waxed my own cars, so never had any interest in the "paint sealer" from dealers.
However, from what I have read its all a bit of a con! Sure, you take the treatment and you get a paintwork warranty - but in all honesty, it should cope fine with the conditions we have here.
If you park under trees, then sap will get on the car and on the paint. Wash it frequently - once a week, and polish it yourself - if you cant get time to do the whole car, focus on polishing the flat surfaces like the roof, boot and bonnet where sap is most likely to stick.
There are loads of good products out there (£10-£15) that will keep the paint in perfect order without costing the earth, and certainly wont cost the dealers "seal" price.