i havent installed it and i wont because Windows update doesnt work for many
but changing tabs is supposed to be faster than firefox, so if all you want is tabs i would go for ie7 but only when the final comes out because this beta at least isnt good enough really.
and atm ie7 is obviously more secure than firefox because none of the viruses and spyware written for ie6 work but as soon as ie7 is released it will go back to normal