Well like all thinks in life you never get 100% what is says on the packet so to speak. but buying expired domains is good as long as you get them quick and use the domain as fowarder or to place links in for ads or park the name. but from my past explots on buy these sort of domains i find it best to try to keep with the currant theme - search on the wayback feature on alaxa.com - as that why the traffic is going to and if changed then over time your see the site drop in traffic and link backs and so on.
But keep with the same theme add your ad codes and links to your networks and see the site stay with the same traffic and may be grow with it taking your other sites with it.
Hope this is helpfull - but this my thoughts and some may say different Q- of the day ((but who is to say i am wrong or right. as there no wrong way to make money on the net only easyer ways and hard ways))