We do letter canvassing campaigns for particular clients (just finished a whopping 700+ letters for one!) as part of the service, if they're after something unique that doesn't come on the market very often, have limited criteria or are keen on a specific area of a suburb.
However, we put them in envelopes and post them

as home owners are much more likely to open them and peruse rather than a flyer popped in the letterbox with the rest of the junk mail. Naturally, it's much more work but it does yield results, when compared to simple flyers (very low hit rate). We also personalize the letters according to the specific client's needs (and situation) and this seems to have been more successful as well.
If you are genuine about wanting a particular area for a PPOR such letters can be effective, even given the low strike rate. We've had people ring us up many mths afterwards, wanting to know if we were still interested in their property. Doorknocking can also work, but you need to be prepared for many rejections and suspicious home owners!
Good luck
