Most of the time I use a Sigma APO 170-500, the old one. There are several things about this lens I really like. It's fairly light-weight, which to me is quite important since I use it as a walkaround/bikearound lens, and often have he thing around my neck for hours in a row. Manual focus is extremely fast on my copy, and the intense zoom creep makes you can actually zoom in trombone-style, which has its advantages (these things might be bugs, but I've embraced them as features). It is very noisy, of course, due to the screw drive (though I've often wondered if the same may not be the case for lenses with a hypersonic drive; it's just hypersonic to us, not necessarily to animals with a smaller head or more acute hearing). Because I pretty much limit myself to urban/suburban wildlife, this probably is less of a problem than in remote areas, where wildlife is much more shy and alert.
Of course, the big problem is IQ on the long end, which with the old model apparently is far more of a problem than with the newer ones. @500mm, my copy performs best at f/14, and anything shot below f/8-f/10 is pretty dreadful. Here in NL, F/10 handheld is doable in spring/summer, but the rest of the time... Of course, part of this comes from the fact there's far more risk of motion blur on the long end, and the fact that I often do serious cropping, because I tend to shoot small subjects at far greater distances than a 500mm lens should be used for.
I made a simple DIY modification with some adhesive velcro to prevent zoom creep, and use with with a shoulder stock whenever possible.
Image examples:
anything wildlife in my photostream on Flickr (i.e., about 90% of the images), unless explicitly marked as shot with a Novoflex lens. And that, of course, is my other one... an old Novoflex Noflexar f/8 600mm with fast-focusing pistol grip on integrated shoulder mount. I recently obtained an f/5.6 400mm triplet head as well, but haven't taken it out in the field yet. The 600mm at least is a wonderful lens to use with a Pentax DSLR. It does "add a few decades" to the images, so unless you like that sort of thing it's not for everyone, but still.
Here's a set; for the earlier ones in it I used a K100D, even.
I also have a Pentax SMC f/4 300mm, which I've tried on its own and in combination with the 1.7x AF converter (I know, it's not supposed to work with an F/4 lens, but still). I haven't been all that impressed with it on its own, and with the converter I get severe front focus. So no joy in that department.
