I got Halfords bike racks which were manufactured by Thule, so check their stuff to see if it's better VFM. I have square bars and in retrospect wish I'd paid the extra for quieter aerobars. Box is a 2nd hand Thule, the shape looks quite streamlined and it's only half width so I can carry the bikes. It's great on holiday but don't put anything valuable in there and I put a long bike lock around mine to discourage thieves. Petrol usage up 15% and speed down to 70-75mph max, so there is a price to pay.

We're not all pygmies, you know...

Adoption. Maybe make a few quid as well!

Carracks.co.uk have good deals on their Thule boxes, plus they do a cheaper range branded as Karrite. These are cheaper and better than the Halfords stuff if it's of any help. The guys there seem knowledgeable and they do next day delivery. Don't know if this helps!

A roof box makes so much more sense than taking a Clio and an Octavia every time you go on holiday.

Following the birth of our second child we sold our large (small in todays world) mk2 cavalier hatch and bought an AX 1.4 D, and said to my parents "If we are going to visit you, you'll need to borrow a cot" - A neighbour lent them a cot for the week.

I've used the same (s/h Merc-badged) roof box for a few years on a C43 estate and then a 211 E class estate. The impact on fuel consumption seems to be surprisingly low - maybe 10%-ish.

The only downside on mine is wind noise above a certain speed and the fact that the sunroof is obscured.

For those that ask "why use one?" the answer is two young kids and long trip which involve taking buggies, cot beds and other bulky items and/or camping equipment.

As another poster says if you ever want to take a bike or two as well a long narrow one could be an idea. And it would be easier to get on and off and to store. Mine takes two to get on and off as it's quite heavy and awkward to position. And don't forget you need somewhere to store it.

The thread has gone off on a tangent but coming back to the OP's original questions:

1. I bought a Kamei Delphin 470 in 2010 for holidays and I am very pleased with it. It is much sturdier than the cheaper boxes and should last much longer. The only problem I have had was opening it the first time. Kameis have a very strong seal and I thought the lock on mine was broken when I couldn't open it. Eventually managed to slowly open it on one corner which then released the seal - no problems at all since then.

2. Mine came as a pack from www.roofbox.co.uk with the Thule aerobars. I can't say if they are quieter than square bars as I have never used square bars. However, I don't notice any wind noise so maybe the aerobars work.

I concur with the aerobars are worth having contingent...

I've got a set along with a Kamei 340K roofbox which will happily go on the roof of my E36 along with a couple of bikes on Cruz Bici-racks.

Allows enough space for 2 adults, 3 kids and a weeks worth of camping kit.

Re economy:
The bars alone don't really affect the overall economy, but they are easy enough to put on/take off and with them on and the sunroof open you do get a lot of added noise and turbulence.
However, adding the box does knock a couple of mpg off the usual 27-30mpg...although this may be reduced if you have it empty rather than full along with a full weight car!