Just to give an alternative take to @mellis903 - I’d say the series hits the ground running and there are far, far, far more hits than misses. The first series gives the Slitheen a much more suitable home than the main show; Eye of the Gorgon deals with dementia in a hugely mature way; and obviously Whatever Happened to Sarah Jane Smith is amazing and probably would be even if this wasn’t a kids show.
Series 2 gives us Mark of the Beserker which is another very mature episode looking at absent fathers; another cracking Trickster story; a fun Sontaran story and Bradley Walsh being super creepy as the Pied Piper.
And I don’t think it ever looks low budget.
If you want other thoughts we did the first series in TV Club Extra a little while back.
TV Club Extra: SJA: Revenge of the Slitheen
And you’ll find even more thoughts in the dedicated thread: