At this exact moment, I'd say it's a bad time to get into it. The difficulty really doesn't matter because there are always new alt coins to mine but the hardware is a different story. I've been piecing together a new rig and the prices of GPU's I've been looking at have jump $30-40 IN A WEEK, and that's if you can even find them in stock. On top of that newegg is limiting the number of GPU's you can order to one (at least on the gigabyte 10 series I just ordered). Everything may change tomorrow but for today you better expect some headaches when buying the hardware.
People - the OP states he already has the GPUs, so most of the concerns you mention don't apply to him. His question is in regards to expectations on ROI for a new miner.
IMHO - if you have already secured the hardware and you paid near MSRP for it, you should mine. Your expectations to just recover costs is very reasonable. Anything beyond that is just a nice bonus until you decide to discard the gear.
You can hedge buy selling half of your coins in 2 to 3 months if prices are still flat - or just sell enough coins to recover the hardware costs.
All cons people write only suggest they never bother to read your post. You have the cards already - either sell them quickly or mine.
Either way you are in good shape as far as recovering your costs. Mining has the potential to give you much greater returns in the longer term.
As far as running a mixture of AMD and Nvidia cards on the same rig, your mileage will vary on that. My setup doesnt support that but i am sure others have that working. Keeping it all in one rig and running it stable is crucial to that ROI.