- weight. The 70-300L is significantly heavier than the other 70-300 lenses, so if you're walking to the top of Half Dome and back in a day, you may want to carry a lighter long zoom lens with you.
Would you like to comment on the weight issue in a way that is meaningful?
The lightest zoom is a P&S superzoom which you appear to be concerned about
Looked up the review http://www.photozone.de/canon-eos/592-tamron70300f456vceosapsc
The build quality is not comparable to e.g. Canon's EF 70-300mm f/4-5.6 USM L IS but it's very good nonetheless
- It is 285g lighter than the 70-300L - not exactly a significant weight advantage.
- it is not weather sealed
- IS is not to the same standard and the VC does not offer a tripod detection
- The lens body is made of quite high quality plastics based on a metal mount.
Verdict - The most interesting question is, of course, how it compares to the genuine Canon lenses in this range. The Tamron manages to stay a little ahead of the consumer-grade Canon EF 70-300mm f/4-5.6 USM IS but it doesn't totally reach the professional-grade Canon L variant (especially in terms of bokeh quality). Even so it represents an excellent value offer in the APS-C market.
So it is a good budget lens which edges ahead of the 70-300 non L
Is it worth the premium for the 70-300L? - well that is a personal opinion