The reason different people have different ideas about whether Inov8s are narrow or wide fitting is because different shoes in the range have different widths. In general the more race oriented shoes and fell shoes are quite narrow, but the more trail oriented ones eg Flyroc are a bit wider. The 315s I'm recommending are somewhere in between. If you look on the website they do tell you which are which – here's the wider fitting ones http://www.inov-8.com/Products-Recommended-By-Features.asp?S=%25&T=%25&RC=%25&LA=324&PG=PG1&L=26&G=%25
I wear out all my Inov8s inside the heel, and also had the upper on an early pair rip apart (orienteering is rather tougher on shoes than that particular model is designed for!) Though I wear out the inside of the heels on all brands of shoe due to heel spurs and to be fair they are very light shoes and durability weighs. 315s are certainly more durable than the lighter racier shoes – I've abused mine to the point I'd probably have made any shoe rather tired and they still work. If you want Inov8s that last then go for heavier models (eg Terroc).
I should mention for those who've missed me saying it before that I used to get free Inov8 stuff, so I'm obviously a bit biased (but it does mean I know a lot about the products, having chatted with the people there). I do have to pay real money now, and will still buy more Inov8 shoes when my current ones really do get too worn out (I think the midsole on my 315s is collapsing, so that may be pretty soon) – also no particular reason for me not to recommend something else if I thought it was better.