I can comment on the Velux GVT154 conservation roof window - having had 6 of them fitted in the last month during a complete slate re- roofing.
Comments are not terribly positive, having had to replace 5 out of 6 when my roofers found defects.
This Velux model has an aluminium framed window with a central mullion, and hinges to one side to act as a means of escape window.
The problem was the plastic moulded surround, 2 had cracked right through in the upper left corner, 3 more had hairline cracks in the same place.
I suspect the problem was a poorer quality moulding caused by using too high a proportion of recycled plastic. The replacements still feel rather more brittle than I would like.
This model of Velux fits by sitting on top of the battens, which must be recessed by 7mm (my roofers were grateful for me running custom sections through the thicknesser for them). Upper and lower noggins are also required and a sliding steel frame arrangement built into the Velux should be screwed to these noggins.
This design would make them more difficult to fit retrospectively.
One other concern my roofers had was the built in lead flashing was thinner than the stuff they usually use elsewhere.
On the plus side, the visual impact from outside the building is very acceptable - they're in a highly visible location and blend in well.
So - overall advice based on my experience - ruthlessly inspect your potential purchase, does it really look like that it will give many years faultless service? What's the warranty like? What's the back-up/ manufacturers support/availability of a local rep like? (Velux were good on this score). Ask how it fits retrospectively (I'm assuming you're not doing a full re-roof). Ask how it fits with your type of roof covering (slates, tiles - thatch???). How difficult would it be to replace a broken sealed glazing unit? (With the Velux I would probably buy a whole new one and swap it off the hinges). Do you have underfelt on your roof? Does it work without underfelt if not? Does it have a gas strut stay open? Will that work on the angle of your roofline? What about strong winds? How would you repair other damage? (With the Velux I think I would epoxy on a patch or flashing repair).
I'm bowled over by the quality of what my roofers have done, particularly their leadwork - with hindsight I would have checked out other window manufacturers and been willing to pay more - but I bought on Velux's reputation (considerably reduced in my eyes) and the fact that I had very tight size constraints on what was needed and the GVT154 exactly matched those.