If you look at Rafa's major signings (over 5m) there are more successes than failures
SUCCESSES
Alonso bought for 10.7m sold for 30m
Garcia bought for 6m sold for 4m
Reina bought for 6m sold for 2m
Crouch bought for 7m sold for 11m
Sissoko bought for 5.6m sold for 8.2m
Agger bought for 5.8m sold for 3m
Kuyt bought for 9m sold for 8m
Lucas bought for 5m still here
Torres bought for 20.2m sold for 50m
Skrtel bought for 6.5m still here
Mascherano bought for 18.6m sold for 17.2m
FAILURES
Morientes bought for 6.3m sold for 3m
Pennant bought for 6.7m left on a free
Babel bought for 11.5m sold for 5.8m
Dossena bought for 7m sold for 4.7m
Keane bought for 19m sold for 16m
Riera for 8m sold for 3.3m
Aquilani bought for 17.1m left for an undisclosed fee
Relative Successes/Failures
Bellamy bought for 6m sold for 7.5m
Benayoun bought for 5m sold for 6m
Johnson bought for 17.5m still here
Maybe you could argue over Sissoko being a success but i thought he did relatively well at first before losing confidence but we made money on him so hardly a big deal.
Same with Crouch really in that he played well but didn't always score as many goals as he should have but again we made money on him when he was moved on.
With the failures Riera had one pretty good season but ultimately wasn't good enough and Aquilani was hampered initially by his injury and then the managers after Benitez left obviously didn't rate him but when he did play he seemed decent to me (not outstanding maybe but certainly not shite).
It looks like with the exceptions of Aquilani , Pennant and Babel we didn't lose out too much on the failures and with the successes we either made money or got our moneys worth. I got these figures from
http://liverpoolfc.wikia.com/wiki/Rafael_Benitez/Transfers so if some of them are wrong then blame them.
It needs to be said also then for some of his time in charge Benitez had to battle Hicks and Gillette with regards to money and there were times when he had to sell players in order to but other ones. I think he would have preferred to keep crouch and maybe even bellamy but he needed to. I haven't included some of the cheaper signings (of which there where many) but it's usually the larger signings that a manager is judged on.
If Rodgers loses his job at the end of the season or whenever his critics will point to markovic , Balotelli or Lallana if they fail to deliver but no-one will mention sahin or assaidi because they weren't as costly.