[quote name='sagu' timestamp='1336987181' post='18191']
Well it has a small size if that is a plus. A 500mm or 800mm lens may provide high camera shake.
[/quote]
Camera shake is exactly the same, just shows up more by the higher magnification <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='
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But for that we use support.
The big negative of mirror lenses is the bokeh. The construction makes for circle like crap in the out of focus areas. This is the same with all mirror lenses.
Bokeh psychedelia:
http://abbazz.zenfolio.com/img/v1/p293409921.jpg
http://jazzandflyfishing.com/wp-content/uploads/2012/03/sigma_mirror_lens_-_white_rod1_of_1.jpg
http://farm5.static.flickr.com/4077/5394380056_55d1a78863.jpg
http://3.bp.blogspot.com/_JuWrrjViyFQ/R9hKC5893MI/AAAAAAAABQU/U1iedbDaDow/s400/IMG_7272.jpg
Who will worry about camera shake when the bokeh is SO dizzying:
http://www.dyxum.com/images/forum/kb/bokeh/181841698-L.jpg
http://www.theriderfiles.com/wp-content/uploads/2010/06/Lawson-Pocono-80-sm.jpg
I do not really understand why you would want a 300mm mirror lens. It is not like the quality 70-300mm lenses are big or unaffordable.... (for instance the Canon EF 70-300mm f4-5.6 IS USM, Sony 70-300mm f4.5-5.6 SSM G, Tamron 70-300mm f4-5.6 VC USD, Olympus 70-300mm f4-5.6 ED... all good quality lenses and quite compact).
Like Alexander points out, 500mm/600mm/800mm mirror lenses make more sense, as 500 to 800mm lenses are very bulky and very expensive. Then the IQ problems posed by the construction together with the lesser weight and costs can be a good compromise.