We use PaySystems at the moment, and most of the time they are ok. PaySystems is much more flexible than 2checkout I hear. You cannot custom packages on the fly with 2checkout. All you plans have to be set out in their system first. So this makes it virtually impossible to have addons.
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2checkout has been very simple and straight forward to use for our hosting solution !
Although I gotta say that the limitation of having to use US dollars has been a thorn in our eyes .
Had a look at worldpay as well and it looks good apart from the initial high setup costs ! The good thing about worldpay is, its able to process in lots of different currencies !