I have used some of their pattern sheets, but purchased these at shows or from them directly receiving the printed paper. Quality is quite nice. For downloading, and more reasonable in terms of cost there is Scalescenes at http://www.scalescenes.com. They have lots of patterns all downloadable and you can print out as many as you need, whenever for that initial purchase. All it costs is toner and paper. File format is PDF and there are no editing options (unfortunate but understandable). In addition to patterns such as bricks, pavement, corrugated metal, clapboard, they also have some kits, including two free downloads. Note there materials are all scaled to 00, 1:76. Printing to H0, 1:87, is easy though. Select A4 for the paper and print at 87%. For more details see http://www.scalescenes.com/hoinfo/hoinfo.html. There is also a difference between UK N scale (1:148 I believe) and North American/Continental Europe at 1:160. The difference in scaling is still 87%...
In terms of working with card models, I have gathered a bunch of references to tutorials as well as having published one myself. Those can be found at http://www.philobiblon.com/eisenbahn/tutorial.shtml. I generally mount my paper on card of the appropriate weight which reduces potential for warpage. That said, I have had very few problems on my various project that can be linked to from my "Scratchbuilding" link.
Here are a few examples. The one at the bottom shows part of my n-scale module with a view of a retaining wall by Scalescenes (as well as corrugated fence and brickwork on the houses) and the germanicized Quonset Hut by Clever Brothers. The small industrial building was a kit that I expanded by kitbashing.
I love working with card. Very versatile. Doesn't have the (exaggerated) depth of patterned styrene, but with weathering and good technique can more than give it a run for its money. I especially like working with photographic images as in the case of the small grill.
