Absolutely Awful - Sente seems much more stable; it doesn't have Cite While you Write (CWYW), but then again EndNote's CWYW crashes virtually every time anyway.
Having previously used EndNote fairly reliably on a PC, I bought it for Mac before the 'X versions'. That version rarely worked, but given the hype about the newest refined X2 version and CWYW fixes, I paid again for the latest update (another £75 or so!). It is every bit as buggy, and has virtually no innovation over previous versions. In contrast, I am now looking at Sente, which lacks the much hyped integration with Pages, but will produce bibliographies for Pages outwith the application itself after inserting easil formatted citations. Sente also features a much more intuitive interface, web search facilities that actually work, and a great system for linking downloaded PDF files to the citation database. It can be a little slow opening my 50k database, but not intolerably. Unlike EndNote, it also has a trial.