I own a SilShelter and tend not to use it. Other than that, I'm neutral.

I mostly find it's too small for my personal preference. I'm Not totally crazy about the door either, but it works. Might also mention that for weird reasons, it's a relatively fussy thing to pitch, though not a big deal after you've done it a few times.

Personally, I never think of a bivvy as a tent substitute. I think of it as a nearly indispensible (for me) sleeping bag accessory.

To my way of thinking, just personally again, I'd never want a bivvy that weighed more than about one pound.

Works absolutely fine to wear clothing inside bag, as long as they're not wet.

You want enough effective insulation to be warm, and not so much that you become hot. That's the ONLY ISSUE regarding warmth. Whether it's clothing, or a sleeping bag, or bivvy, or some combination, that isn't relevant to sleeping warm.

Since temperatures are generally coldest at night, and one produces least body heat while sleeping, one makes most efficient use of total insulation poundage by wearing all clothing to bed. That way, you can carry a somewhat lighter sleeping bag.