Yes, I have one of their summer houses as a shed. Its OK, far better than a shed would be, but I would hesitate to use it as an office. I have a small (4m x 2.5m) one in 28mm timber. The back wall bows like buggery when you put it together - whatever the instructions say make sure you screw down the first course ot the bearers all the way round, not just at the corners. If you are going for the 44mm thick one it may be better.
I don't have an internal wall and insulation, which I think would be essential if you wanted an all year round garden office as opposed to a shed. If that is not included in your cost then you might want to investigate the extra cost. To be honest, if you can see a summer house near you for comparison purposes then I would highly recommend it, just to get a good idea of the structure. The other thing to watch is that they recommend you do not attach stuff to the internal walls, since the planks expand and contract - by a considerable margin according to the blurb.
In terms of the build, it was a doddle - just slap the planks together and tap with a RUBBER! mallet - the cutouts were OK. The whole structure was a bit flimsy until the roof trusses went on, which stiffened things up a lot. I would say a 1 man build until eaves height, then possibly a 2 man build depending on the style of the roof since the apex comes pre-made as a large triangle and can be heavy. Roof was OK to fit on my own.
Watch the roof covering - they do not include it in the price. In the end I sourced EDPM membrane from elsewhere, which has a 50 year lifespan and is much better than felt. I used contact adhesive, but that was a big mistake - get water based adhesive is my advice as covering the roof, even with 2 of us, was a real pain and there is no margin for error.
Painting - I went with Sadolin super-dec in the end - not cheap, but worth every penny as it flexes as the wood expands and contracts, unlike gloss paint which will crack.
Steve