I've always used a tube red dot or holo sight, but am interested in a reflex red dot for an upcoming CQB build. I'll be honest, they look pretty cool, and I like the idea of a simple piece of glass in my line of sight. But, nobody seems to run them (outside of 12 yr olds on CoD). So, hivemind, what's the story? Are there any good ones for around $100? What are the pros and cons? At a glance, they appear less durable than other designs, but I'm no engineer.

If this has been covered well, please point me to those threads; search sucks for me.

Trijicon RMR is no toy.  Nor is the Leupold Delta Point.  Nor is Dockter or J. Point.  No, they are not $100.  Those are toys.

I plan to buy a Delta Point 2 when they are released in mid September.  About one ounce in weight.  About $400.

The RMR is super light but I really don't like the small window or the amount of distortion I get when looking through one.

The Delta Point is crystal clear with no distortion.  I don't have one because it loaded the battery from the bottom, like some others, and depended on posts in the mount to maintain zero when removed for a battery change.  Also, it had no true "off" button to deactivate the motion circuit for transport.  That circuit automatically turns it on when you pick up the rifle, and off when stationary for five minutes.  Great feature, giving it always ready capability and years of battery life.  I'm going to get the Delta Point 2 because they fixed both:  battery now top loads and it has a true off switch.

OP:  Leupold owns Redfield.  The Redfield Accelerator is out.  It is a less expensive version of the Delta Point 2. Same basic design, size and weight.  While it may lack the smoother lines of the DP2, and might be not quite as rugged, it is getting pretty good reviews and is probably good for everything short of combat duty.  It is about $200.

So it seems that at the price point I'm looking at, most of what's available are really more toys than anything? There is/was a BSA model with a circular glass piece that looked a bit more developed; that worth looking into?

No, stay away from the BSA, Barska, Ncstar stuff. It is pure garbage.

The most important things to look for should be decent parallax, zero retention/good mount, and a tight reticle

No.  All those sights in that style and under $100 are just rebranded, relabled stuff coming out of the same low quality control Chinese plant.  I tried one sold by Sightmark on a .22LR intended for a child member of my family. Sightmark Sure Shot.  They are all the same, though.  Four reticle choices.  All shot to different points.  Very difficult to sight in (no clicks, and no uniformity of movement amount).  Then would not hold zero.  Bumping the rifle destroyed zero, even with the adjustment screws locked.  The mount and cross bolts were made of very soft low grade "pot metal."  Battery life was terrible.

Save up for the Redfield Accelerator, or stick with one of the basic but reasonably reliable micro dot "tube" type red dots, like the one by PA, or the Bushnell TRS-25.  Those are in the 4 to 5 ounce range and reasonably compact.

Burris Fastfire III?

MePro M21?