[uvex]

Has anyone used Canon EF 100-400mm f/4.5-5.6L IS USM with
Canon 450 DSLR? Does it work well with 450D?

Are there any Tamron or Sigma lens which is similar to
Canon EF 100-400mm f/4.5-5.6L IS USM?

What is the 2nd hand price for Canon EF 100-400mm f/4.5-5.6L IS USM?

Thank you for your help.

[Obi-Wan]

"Has anyone used Canon EF 100-400mm f/4.5-5.6L IS USM with
Canon 450 DSLR? Does it work well with 450D?"

big white can be used on any EOS, my experience: it can be used without any problem with 400D, should be ok for 450D (no 450D to try)

"Are there any Tamron or Sigma lens which is similar to
Canon EF 100-400mm f/4.5-5.6L IS USM?"
yes
Tarmon
http://photo.net/equipment/tamron/200_500_Di/
Sigma
http://www.pbase.com/cameras/sigma/apo_120_400_45_56_dg_os_hsm

"What is the 2nd hand price for Canon EF 100-400mm f/4.5-5.6L IS USM?" at least $8000

[fish30d]

The only problem you may encounter when using 100-400 on 450D is weight imbalance; 450D weights around 600g while 100-400 weights around 1400g!

[uvex]

Really 100-400mm weights so much! I see! Thanks!

[Canon Ball]

Since all the CHing answered your questions.
If you know you will make use of this long range.
I can only say, you will never regret buying this Len.