tl;dr Fluid gameplay, excellent and crispy movement, fast paced combat similar to Megaman Zero games, challenging but not unfair.
Downsides would be lack of exploration, I never felt like I ever got lost. Also some platforming sections are brutal with no room for error.
Let's talk Silksong. Having completed the game (and the OG HK), I can say without any doubt that this is a step up from its predecessor. Hornet is such a mobile character that she plays completely different from the Knight. I started off playing like the Knight, calculated and careful, and got slapped by many of the early encounters.
Look, this game is NOT designed to be played slow. It punishes mistakes heavily (the 2 damage "issue" that many have complained about), but this is NOT a design flaw. The Knight heals a single mask with 2 seconds charge, Hornet heals THREE, and she can do this in mid air! What this means is you just need to go in swinging, get as much silk as you can and heal up as soon as you see an opening. Once I switched to this playstyle, none of the bosses became an issue, many of them took one try to beat. Get used to aggressive pogos and dash attacks, because they will be the greatest source of silk (read as heals).
Boss difficulty is excellent for most bosses, some of the lategame 1v1 bosses are fast paced and intense. People who say there aren't any tells aren't using dash to react. Dash gets you out of an attack zone very very easily. Dash out, dash attack in, rinse and repeat. The thrill you get when you narrowly dash out of an attack is AMAZING. A couple of bosses are a bit unfair though, some have strange hitboxes and others just want to waste your time, teleporting all over the place and backdashing to avoid your attacks just because they feel like it. Why have this RNG mechanic in boss battles?
Exploration is pretty fun, there are tons of zones with diverse sceneries. One issue is that they are rather linear. You will not be getting lost at all. Branching pathways are usually short and some punish you with a crappy trap or spike zone. It gets quite ridiculous when you decide to go exploring and fall into a dead end pit three times in a row.
But with exploration comes platforming! And boy is the platforming in this game brutal as hell. You got your additional mobility, let's make sure you use every single iota of it. And mind you, these sections are NOT OPTIONAL. You will have to get through them to complete the game. They demand so much mechanical skill that casuals will be completely shut out from even progressing. Honestly platforming sections can be harder than even bosses.
Story is cryptic, music is amazing, but those are what we've come to expect from HK.