Please read the release notes at https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/master/docs/release-notes/release-notes-0.9.1.md for details on what this release includes. This release fixes some errors in how timelocked utxos are managed and spent; be sure to read the first few sections of the release notes about that. Also it updates some details in the documentation to be more detailed and helpful, so you're advised to read through those changes.
Apart from that there are a few other minor changes or bugfixes.
You're strongly recommended to backup, and then recreate your joinmarket.cfg (in ~/.joinmarket by default), to make sure you have the latest version of the config settings. Then read the above release notes which will guide you on what changes may or may not be needed for your situation. Note that some users, but not all, will want to recover their existing wallet into a new fidelity-bond enabled wallet - the main section of the release notes will guide you on this.
Please verify the PGP signatures of the releases (zip files and exe file, see below) before using. They are signed with AdamISZ's pubkey (fingerprint: 2B6F C204 D9BF 332D 062B 461A 1410 01A1 AF77 F20B.
A reminder that since 0.7.0, (gpg signed) Windows executable builds of JoinmarketQt are included with the release.
(Similar binaries for Linux and Mac are not provided but may be if people show interest in testing and working on it.)