Sarah Jane's a good companion. Yes, she has a tendency to wibbly-lipped weeping at the merest hint that the Doctor might be dead but she's brave, feisty & a damn fine journalist. You can see why she'll end up in her own series. There aren't many Doctor Who companions you can imagine doing that, even popular ones like Jamie. They need the Doctor but because Sarah Jane was given a life outside UNIT from the get go, which allowed her to drive her own stories she's got the strength to carry on a fictional life away from the Doctor.
This is down to a combination of good writing & perhaps most importantly Liz Sladen's performance. Barry Lett's said that one of the reasons he cast Liz was because she could do 'fear & bravery' at the same time, which is actually one of Sarah's key strengths. We know she's afraid, sometime terrified, but she - just about - holds herself together so that she can help her best friend, The Doctor. Liz worked well with Jon Pertwee but with Tom the companion-Doctor relationship reaches something of a peak in my opinion.
There's one major piece of silliness involving the RAF - using stock footage as used in Seeds of Doom - firing some kind of nuclear bomb into a nuclear power station as a way of stopping a creature that we've already established thrives of radiation. Never mind the ridiculousness of the fact that driving about five miles away seems to be a 'safe' distance from the site of what is about to be the first nuclear bomb dropped on British soil.
Whether that kind of quibble stops you enjoying the story is entirely up to you really. I know some people can't stand plot holes but I'd say there's rarely a Doctor Who story without one of them, if analysed deeply enough.
In the end this isn't up to the standards of Season 13 or Masque but it has its moments. It is also a lovely farewell to a fine companion.