To throw out a dissenting opinion here, I do not think they are worth watching.
IMO, they are simply not well-written. Their plots are incredibly contrived, and their climaxes always descend into one big magic hoopla where anything goes, regardless of whether it makes sense or not. In fact, if you watch them, the first thing you need to understand is that magic in the series can basically do anything, and has no limits or restrictions whatsoever. It does whatever the plot needs it to do.
The films utterly fail at juggling seven main characters, and as such, I do think that the humane five qualify as main characters at all. All development in these films go towards advancing Twilight and Sunset Shimmer, and the rest of them receive none of it. They are mostly just around to fill space.
On top of all this, the films are not even remotely relevant to the show. They exist in their own little isolated bubbles, and the events of the films are never referenced anywhere in the show, nor is there even the slightest indication that they are even canon to it. You will miss absolutely nothing in the show if you were to skip them.
Others will tell you that Sunset Shimmer alone makes the films worth it. Honestly, yes, Sunset is the only thing that the films have going for them. However, if you're like most other Sunset fans, all she's going to do is make you want her to appear in the show, and it's very unlikely that it's ever going to happen.
Just my opinion.