The thing nobody tells you about a party outfit is that the real test comes three wears in, not the first one. That's when you find out if the sequins are glued on or stitched, whether the zip still does up without a fight, and whether the fabric that looked opaque under shop lighting has gone slightly see through under a flash photo you didn't ask to be in.
A cheap party piece tends to look its best for exactly one evening and then starts giving up the ghost almost straight away. Sequins come off in your handbag and turn up in the bottom for months. Metallic thread goes dull after one gentle hand wash, never mind a proper one. Lining, if there's any at all, bunches at the hem and makes the whole thing cling in a way it absolutely didn't in the changing room. A well made version, whether it's a slip dress, a sequin top or a tailored jumpsuit, uses fabric with a bit of weight to it, has sequins or beading sewn through rather than glued on, and finishes seams so they don't fray the first time you sit down in the wrong position. Hand wash or dry clean where the label says so, store sequinned pieces flat rather than on a hanger so the weight doesn't pull the fabric, and steam rather than iron anything with embellishment, because an iron and sequins is a bad afternoon waiting to happen. Fit is where most of the disappointment lives, and it's rarely about size so much as proportion. Slip dresses need checking at the bust for gaping, bodycon needs checking at the hip for a line that digs in rather than skims, and anything strapless needs sitting down in before you commit, because what stays up standing at a till point often doesn't stay up once you're folded into a chair for three hours. If you're between sizes, go up rather than down on anything structured, since a dress that's slightly loose through the body reads as deliberate, while one that's tight in the wrong place just reads as a mistake.
The useful party pieces are the ones that don't insist on being worn alone. A good sequin top goes under a plain blazer on the way there and comes into its own once the coat's off, a slip dress works over a roll neck earlier in the evening and without it later, and a decent pair of wide leg trousers in a dressier fabric will do a party and then quietly go back to being trousers for something far less exciting the following week.