For my one-man office, I use a Xerox WorkCenter 7120 for 11X17 color plots and am very pleased with the results (very fast, two sided when needed, very high quality). It was more than a little pricey ($7k), but I've been really happy with this color printer/copier/fax machine/scanner.
For bigger stuff I use a 17-inch Canon Image Prograf 5100. Again, pretty expensive but I defined a "half-D" and "half-E" paper size so that I can use Acrobat and tile D and E size pages onto two panes, connect them with glue stick, and can hardly see the seam. I just didn't have floor space for a D- or E-size plotter and this has been a great compromise. Print quality on photos is better than studio quality and I do a lot of that (photographs for framing and using the roll paper for long panorama scenes is amazing), if you don't print photos then this printer may not be worth the money to you.
David