Installers in our area were so heavily booked that scarcely any would even reply let alone provide a quote, so a self-install was really the only option for us. The design service and especially the great after-sales support made this possible, and now our stove is up and running, ready for power cuts, gas shortages and anything else the winter throws at us.
That said, it's important not to expect to much from the service. It cannot and does not account for the idiosyncrasies of your property, e.g. do you have adjust the angle or length of the offset section to accommodate joists/rafters and the tiling arrangement on your roof? You still have to do lots of research to really understand the design and the kit you've been given, and to have the necessary tools and (minimal) skills to use them, e.g. if you're going to have to cut tiles, make sure you've access to a grinder and plenty of cutting discs! Also plan your work carefully so that any on-roof stuff can be completed within any rain and frost-free gaps the weather gods see fit to offer you. Don't be alarmed if you end up having to swap out / return components because you need to depart from the plan slightly; the Stovefitters' delivery service is very fast and efficient, and the returns service is good, though you may have to give them a little nudge to make sure everything is accounted for.
Overall, I'd say great kit, great service and great value, so long as you're up for the work you'll need to put in.