They are supposed to be very good. However, I was getting my Senator windows through a rep as opposed to a retail outlet until I discovered that the rep in question was telling my builder what size to make all the openings! I reckoned it should have been the other way round so I went with a completely different company instead even though they were dearer! If you can find one of the approved dealers that doesn't have a retail unit with all the associated overheads then you will get them cheaper - just remember my experience!
Spicey
I wouldn't rush to judge in relation to who specifies the sizes. A lot of companies will recommend that you stick to standard sizes
I am of the opinion that it is better to get the window company to measure up and make the windows & doors to fit the openings rather than the other way around. Therefore, if there is any problem the window company would have to sort it out - as opposed to the builder having to make any adjustments - which could cause further problems! Much easier if you are building a one off house I think though I'm sure a lot of people have made their openings to a standard size. It would be interesting to hear from anyone who did this and whether or not it all went smoothly!