In much the same way, Supergirl reflects the majority of the comics from which she spawned; not well thought-out, poorly written with a forgettable villain. There's a reason the Supergirl television programme mines the history of Superman rather than its titular character, Supergirl as a comic character has never really hit in a big way. Her canon is nebulous in the extreme, she has no iconic villains and her character changes drastically version to version. With all that in mind, I do think that this Supergirl film is about the best version that could have been made in the climate in which it emerged. The reference to Superman being in "deep space" at the time of the events in Supergirl curtails questions about Kara's cousin and, retroactively, fits in nicely with the plot of Superman Returns. Although Selena is a crummy villain, Supergirl having to take on Magic (another major Kryptonian vulnerability) rather than Kryptonite creates a nice point of difference to the Superman films, and the fact that Ethan is a gender-flipped version of the typical film "trophy" female is still really clever and doesn't get enough credit.
Peter O'Toole is utterly wasted here, and clearly drunk in every scene he's in, which is funny, but hurts the film a great deal; Brando was amazing in Superman: The Movie despite not learning his lines ahead of time. Helen Slater isn't at all interesting in her title role, I'm not sure if it's down to direction or the desire to paint Kara as a "Magic Pixie Dreamgirl", which just kind of irks me to see, even forgiving the thirty year's distance. Unless you're watching the entire six-film Superman "series" or, like me, am a completionist for the genre, Supergirl is probably best skipped. There's not really enough to hold the interest and the parts that may be useful in planning supers games are covered here and in other reviews.