been meaning to foundry pill myself, found a little energy after NFT NYC.
my main concern was that we are doing a lot of extra things around meta-transactions, signatures and custom errors to support our cross-chain token gating solution.
0xEssential offers a client SDK for our meta-tx solution that we also use in our hardhat tests.
Those tests end up being a bit more integrative - there's a lot of value in a more e2e test that covers our SDK and forwarding contract and context primitives in an implementation contract. But I'd heard good things about Foundry and wanted to give it a shot.
My main takeaway is speed - much faster than hardhat. I think I'd have been more happy using Forge while developing our contracts SDK as well - copying source into lib makes it a lot easier to make tiny changes to the SDK contracts compared to using an NPM package.