The first week of September is when a slip dress properly comes into its own, that stretch where mornings need a layer but afternoons don't, and you want something that looks like effort without feeling like it. I pull mine out the minute the evenings start drawing in, because on its own in July it's either too bare for my liking or too clingy against bare skin in the heat, and neither feels right. Layered, though, it turns into something I can wear from a daytime errand straight through to an evening thing without changing.
Running the usual checks, a slip dress fails on its own almost every time. Spaghetti straps, a cowl or scoop neckline that dips lower than I'd choose, a hem that often stops at the calf rather than the ankle. So it needs a long sleeve top underneath, something fine enough not to bulk out under satin, and that's where it either works or looks like a mistake. Thick ribbed jumpers underneath fight the drape and the whole thing sits wrong, stiff where it should fall. A fine long sleeve layer in a similar weight fixes that, and if the hem's short of ankle length, a straight maxi skirt worn alongside rather than under does the job without looking like a fix. It comes together properly for an autumn evening, layered with ankle boots and a coat that doesn't fight the shine of the fabric.
Where it falls short is obvious once you've worn it a few times. On a humid day, two layers of fabric where one was designed to be worn is uncomfortable, and I've stood at a bus stop in October regretting the extra top before now. It's also not a dress for anyone who dislikes the slight cling of bias cut satin across the hips, because no amount of layering changes that line, it just adds coverage on top of it. And on windy days the fabric catches and moves more than a heavier dress would, which is fine if you're prepared for it and mildly irritating if you're not.
The difference between a good one and a bad one is mostly in the cut and the weight of the fabric. A proper bias cut hangs cleanly and doesn't ride up when you sit down, a cheap one pulls at the seams and rides up your thigh within an hour. Good ones use a weighted hem so it doesn't flap about, and the straps are adjustable without being fiddly, not the kind you're retying by midday. A slip dress done well is one of the easier things to layer into something that covers properly without looking like a workaround.