Martins are undoubtedly the standard by which other guitars are judged. However, it is interesting to note just how many of the 'big name' British folk/celtic guitarists DON'T play Martins. This is a quick list of the artists (all nationalities) I've seen in the past 12 months, together with their guitar make:

Bert Jansch - Yamaha
John Renbourn - Bown
Clive Carroll - Bown
Ralph McTell - Yamaha
Martin Simpson - Sobell
Pierre Bensusan - Ryan
Martin Carthy - Martin (his 'other' guitar is a Fylde)
Davy Graham - unknown (Woolworths?)
Tony McManus - Kelday
Kelly Joe Phelps - Martin
Keith Hinchliffe - Brook
Michael Chapman - ? (not Martin)
Gordon Giltrap - various (not Martin)
Thomas Lieb - Lowden

I offer this as an observation only. You may draw your own conclusions.