Hi Enda,
I used to work for Market Simplicity and this was our main business. It is effective but you need to have a realistic expectation on the figures that you will achieve.
Off the top of my head, the realistic figure for read emails advised by the DMA is 3-5%, now you can achieve higher if the data is targeted correctly and is made relevant to your industry.
When working for MS, we averaged 5-7% for our clients and the ROI was pretty good as achieving a few sales usually paid for the campaign and genereate some profit as it is a relatively cheap marketing tool.
Hope this helps.