I go both ways on the issue.
A used reg equal in performance to a new one can be found for less than 1/2 the retail price of a new one and that can mean $300 in savings. Parts cost on a first second and octo can run $60-$70 per year depending on the model and the percentage of dealer markup on the parts, so in 4-5 years you break even on the costs either way. (cheaper regs like the Mk 2 R190, have lower parts kit costs so the 4-5 year thing still holds true.) So..if you swap regs every 3-4 years anyway there is no real advantage to ever buying a new reg. If you plan on owning it for a decade, it makes much more sense to buy a new one.
This can be an important consideration for a new diver wanting their first reg. If they are cash strapped and need a bargain, it makes more sense to buy a used high performance model than a new lower performance model as the price will be about the same either way and the used high performance model will holds its resale value where the new reg will not. On the other hand if the new diver has the cash and plans to keep the reg through a long diving career, buying a new high performance reg up front from an authorized dealer to take advantage of free parts and warranty benefits makes the new reg a much more attractive purchase even with much higher acquisition costs.
Doing your own service work may or may not be cost effective. Labor runs about $75 for a first, second, and octo and that is what you'll save each year. However doing it properly requires some basic hand tools, some specialty tools and some test equipment. The hand tools and specialty tools can run $200-$300 with another $200 for IP and magnahelic gauges. So again it will take 3-4 years to break even on tools and the costs of parts found on e-bay will run about the same to a little more than what you'd pay at a dealer who uses a 100% markup over dealer cost. So for the DIY approach to make sense, you have to have some mechanical skills in the first place and then ideally you have more than one set of regs to support (technical diver, family with several divers in it, etc) so it is not the answer for everyone.