Mél Bonis composed highly refined salon music, and this Valse is a masterpiece of late Romantic lyricism. It is characterized by sophisticated chromaticism and a subtle, graceful flow.
Achieving a seamless connection between inner voices while maintaining the dance pulse is the main challenge.
Boston-valse (Op.119) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Boston-valse (Op.119) lasts about 4 minutes.
Boston-valse (Op.119) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.