I've made my way through most of this thread... congrats to all the folks who have made it work successfully. You must actually find it unfortunate that pair trading has become a fad, because the more exposure, the less profit opportunity as the market becomes more efficient.
My thinking on pair trading is this:
a) The "easy" opportunities are gone, and have been disappearing over the last few years. I have talked to someone who worked for a fund-of-funds group (allocates to hedge funds) - he commented that many stat arb / quants have been having a hard time lately. Simply because, now anyone can download a price series and calculate correlations & mean reversions in Excel, provided they have specific knowledge. Back in the 1980's it cost thousands to get the kind of data now on the internet for free. This sounds like it corresponds to the experience of many individual pair traders as well (decreased performance).
b) Given that the "easy" opportunities are gone, pairs trading is just like any other style of trading that is widely employed - eg momentum - in other words, it is *hard* to find an edge. Various strategies have their day in the sun, go away, come back, etc. The long term results of trend following CTAs bears this sort of phenomenon out...
c) I don't think I could ever bring myself to have a trading plan that requires averaging down. I know some people do it - and not saying that it will happen to all - but I think the risk that one day you get hit hard with a really big loss, that wipes out many small gains, is there (statistically, it is a strategy with a negative skew). I think new traders find pair trading appealing because it dangerously implies you can avg down with abandon, which is generally a trading no-no. (To confess, though, it's not like I have never succumbed to the urge to avg down - just that I rationally know I shouldn't)
d) Therefore, the only type of pair trading I could consider (and this is a personal statement until seeing someone's account record converts me

) is one where there are stops and no averaging down is allowed. It is fine to trade countertrend with this, same as one can trade a straight price series countertrend. It is just that you define the loss point as somewhere nearby, past support or resistance. But personally, I would rather pair trade using a trending relationship, where the benefit is that the market risk has been taken out. I would definitely consider trading stocks in different sectors against each other, or even baskets of sectors - eg the SMH vs some other ETF...
Just my thoughts & I welcome any feedback...