See release notes at https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/master/docs/release-notes/release-notes-0.5.1.md
Please verify the PGP signatures of the releases before using. They are signed with AdamISZ's pubkey (fingerprint: 2B6F C204 D9BF 332D 062B 461A 1410 01A1 AF77 F20B.
A few notable things: labels on wallet outputs; Python3 now default, installation has a help menu, there's a native segwit wallet (BIP84, bech32) in the backend (but not active in Joinmarket for now). A variety of minor bugfixes, all listed in release notes.