Craigan, I would of agreed with you up until last month! Have you checked out TapeACall (http://bit.ly/ZedFlg)? Once the app is purchased, there are no minutes to buy, credits to purchase, etc.
It's real simple to use, can record incoming AND outgoing calls, and allows users to record an unlimited number of calls for an unlimited period of time. Much better then having to use a cumbersome piece of hardware!
You can download the iPhone recorder for your iPhone 4 and install it to your phone. The app is very easy to use since it just pops up whenever you make an outbound call or whenever you receive an inbound call on your iPhone 4. The recorded conversation can then be saved and sent as an email attachment.
None of the solutions above work to record both incoming and outgoing calls. There is only one solution that doesn't have any strings attached, and it's an actual recorder that you plug into your iPhone: