I bought these mostly for night and dawn/dusk cycling; I use my normal sunglasses for daytime hours.  My quick synopsis is that these are a great value for decent quality glasses that you can use for sports without worrying much about them (like I know I would with a $200 pair of Oakleys).  I like that they come with a hard case and a soft microfiber pouch; I don't use either for my everyday sunglasses, but I do for my cycling glasses that get tossed around in my gym bags.

Build Quality
They're not built like a $200 pair of glasses, but they're not $5 swap meet garbage either.  I think the quality is about what you'd expect for the price.  The hinges are decent but not robust.  The plastic frame flexes but I'd consider that a plus for sport glasses.  I wear mid-range sunglasses (e.g. SunCloud) and these feel like they're in the ballpark but maybe a little flimsier; although I have never owned a pair of open-frame exchangeable-lens glasses before, by nature of design they probably feel a little flimsier.

Lenses
The lenses are just ok.  That's not to say that they're bad, but I don't expect they will be too durable; just plain plastic.  I got a clear, orange, and brown set of lenses, and turns out the orange work well for night riding even though they're a bit darker than the clear.

As others have noted, removing the lens the first time is very difficult.  I ended up doing something a little different from the directions.  I pinched at the nose as said but I grabbed the lens more in the middle of one of the eyes and separated it from the frame there first, instead of over the nose.  After you remove the lens once the process becomes easy.

Sizing
I originally tried a pair of Tifosi Tyrants but they were way too tight on my head.  The Tifosi website said that the Slip is 10mm wider so I gave it a shot.  It feels much wider than the Tyrant.  I'd say I have an average to maybe slightly large male head and these feel great.  The arms are hardly pressing on my ears so I would think these glasses could comfortably fit a fairly wide head.  I pretty much forget I'm wearing them when riding.

Cycling
The glasses are really easy to get on and off with a helmet on but stay put when they're on. The lenses conform to my face and are wide enough to keep out debris when riding through dust and what not -  yet they vent enough that I haven't had any trouble with them fogging up.

Update:
After a couple months of use I have a few comments.  First is that one of the nose pads came off after a few weeks and I lost it.  Tifosi sells replacement pads, and even comment that you should install them with some superglue.  Sounds like they know that they pop out easy - would have been nice if they followed their own advice and installed them originally with some glue.  Moving down to 4/5 stars, but still very happy with them.  They are very comfortable, and I use them whenever I'm on my bike now, not just when it's dark with the clear lenses.