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I have double glazed sash windows, with some fancy coating and gas filled. They used the original wooden frames so you can't even tell they are double glazed (my building is semi protected because it's some sort of poor-mans-listed). Think they are around 10 years old now and no issues, I can try and dig the companies details up... although I have a feeling they were pretty expensive (I live in permanent fear of breaking one), the previous owner quite proudly mentioned the company was on Grand Designs.
chrisbmx116-
"Drive-by listing"? A lot of buildings got listed years ago with a cursory inspection from a passing car hence the term.
The DG units I think you're referring to are slimline. They have a smaller gap which is compensated for by using 3 gases which have better insulating properties than the standard gas and the glass treatment reflects something (UV maybe) that improves their U value. They were far more expensive 10 years ago but they're now available from most glazing suppliers and they cost more but not an extraordinary amount more than normal units. Manufacturing quality of normal units is very variable and often the more unusual options are only manufactured by the higher quality manufacturers and cost more. It's very difficult to break a toughened double glazed pane, much more likely that the seal will fail and you get moisture in them before you break the glass.
I've been looking into this kind of window conversion using slimline units but it's not that economical for a lot of the sashes I work on as they have up to 18 panes in the upper sash. It increases the cost dramatically as the units have a minimum manufacturing cost. It also doesn't have a massive acoustic improvement as the gap is important in cutting down the sound waves through reflection.
The other problem with DG acoustics is once they are open they're not much better than normal sashes, and they are usually open in summer when your pissed neighbours are staggering back at 2am or there's a big party going on over the road. If you live next to a busy road then I feel sorry for you but heavy curtains will still give you a better nights sleep (even with double glazing).
Airhead
Although that is the modern way to match the original sash pattern and the glazing bars themselves can't cause a problem with the double glazed unit, the frame still can, and it's the lower part of the window where most of the water that runs down the glass collects which always rots first. There are well known best practices for bedding in and sealing double glazed units in wood, and the trade is improving because it's more popular but lack of a proper maintenance schedule will lead to them rotting and can lead to the double glazed unit being damaged. Then the whole DG (sorry but it gets boring typing double glazed all the time) unit needs to be replaced which involves disturbing all the beading then properly sealing them again. Unlike a normal pane of glass where the putty hardens and drops out, worst case scenario the glass has to come out, normally it's just a repair to the glazing bar, re-putty and paint again.
Having said all this I made a back door for my own place in hardwood with double glazing. I left a 5mm gap around the dg unit using plastic blocks and sealed the beading inside and out with an appropriate sealant. I'll also be able to keep an eye on the paintwork, especially on the beading. It's also easier with doors because access is generally easy.
There are new types of dg unit arriving at normal glass outlets which can be thinner than the old standard 4/6/4 which should allow more direct replacement sashes to be made. However you still have to counterbalance the weight. Having given the subject a lot of thought I am still recommending weatherstripping and insulated curtains to my clients.