One factor that I think should be considered is asking yourself how much are you currently paying on an annualized basis to upgrade to newer versions of CS and LR when they are released.
In my personal case, I checked my Adobe order history and since Sept 2010 my total pre-tax outlay on upgrades to CS5 then CS6, and to LR 4 then LR5 has been $485, or ~ $160/year. So assuming that I was to maintain such an upgrade policy going forward (if it was still viable which is not the case for PS), and assuming the monthly price remains at $10, by going the rental route I could conceivably save $40/year. In other words a minor cost savings over the current perceptual model, and therefore a positive factor in my ultimate decision making process (It was not even close to break-even proposition with the offer from a few months back for the $20 or $30/month options previously for PS CC (and if I recall correctly, included a host of other CS programs I do not use)).
The inability however to opt out and keep running the software at whatever version you were up to at the time would remain as a negative factor to consider in the decision to opt in to the rental program. Is it enough to sway me one way or the other? It's getting close now and I could see myself going either way on it ...
But in my case, I just upgraded to LR5 last week so at this time I will continue to sit tight with what I have and see where things lie in a year or so.