I used one and I would recommend it. The thing with Epi-NO is you never know if you didn't tear because of the Epi-No of you didn't tear because you were not going to anyway.
I have a chronic painful condition called vulvodynia ( won't go into but it causes extreme pain to the vulval area realted to pelvic floor muscles being over contracted). I was PARANOID about tearing and was willing to try ANYTHING to avoid it. For me to do perineal massage was not possible but I was able to use the Epi-No taking it very slowly. I could only get to 7cm in the end and I had to start it earlier then most women would and take it slower but I really think it helped. I ended up with two small grazes but I also had a waterbirth and I am sure this would have also helped to prevent any tearing. DD was 8.1lbs so not on the small side.
My midwives supported my use of it and the reason I decided to splash out was I was in contact with another women with vulvodynia and she raved about it and I also posted on a forum asking for opinions from women who had actually used it and what they thought and they were all positive - although I have read of women tearing who have used it).
I think it is often the cost that makes people question it's use however in my case it was money well spent and I will use it again for any other children I have.
You can get them on Ebay sometimes for a lot cheaper if that kind of thing doesn't gross you out - they are all sterilised etc, etc.