Kostas, I have a Tamron SP 300/5.6 (Model 54B), and it is one of my favourite lenses. It is relatively small, very sharp, and easy to focus, despite its relatively slow maximum aperture. It is reviewed in Alex's Gallery home.att.net/~alnem/html/3rdpartyprime.html. I rate it as an excellent lens; see SPLOSdb at www.jcolwell.ca.
I bought mine from B&H in Nov 2002 for about $150 US [E+]. Two sold on eBay in Feb 2004, for $75 [E+] and $90 [new], it has been recently listed at Kenmore for $100 and two are currently listed at Adorama for around $200 [E, E+]. These prices are from SPLOSdb. Jim www.jcolwell.ca
