I'm a huge fan of the franchise, and even I hated The Quickening and the Source. I always lodge the failings of the franchise in its creator's desire to move the story of the franchise forward, which is a good thing; the BAD thing was this desire manifesting in some weird post-apocalyptic story about redemption. The Quickening, the Source, Endgame and the aborted ideas for the TV series' sixth season were all about this; only The Search for Vengeance does the idea justice and IMO it's because the original producers had less to do with it. Endgame had the TV series producers also heavily involved, and that's why it was marginally enjoyable for me. Perhaps not surprisingly, both Engame and SfV had extensive historical flashbacks, while Quickening and Source did not.
Highlander was at its best when examining the differences in how people do stuff today versus how they did stuff in the past, using all of history as a rich background. This, plus some truly great character work in the first movie and series, is what makes the franchise memorable (well, this and the sword fights). I hope that eventually someone will reboot Highlander as a miniseries on some cable network, where we can tell a concise, character-based story over many hours instead of two in the theatres, and where the violence that defines this brand of immortality can be graphically shown (instead of off-shot beheadings, etc.) to illustrate how awful AND awesome it can be.