As the first - and last - two-part Doctor Who story for some considerable time, 'The Sontaran Experiment' works well. Despite one or two tenuous plot points, the story generally holds together well, and with its themes of torture and sadism it continues in the adult theme established by 'The Ark in Space'.
Firstly, I'll just address the aforementioned tenuous plot points. The only real one is the ease with which the Sontarans back down when the Doctor tells the General that humanity is ready for their invasion fleet and will destroy it; this is undoubtedly due to time constraints however, and the script does address it by noting that the Sontarans are extremely methodical (and they're undoubtedly fighting the Rutans on another front, so perhaps it does make sense that they dare not risk it). The other weak plot point isn't actually an issue in my opinion, but is mentioned in The Discontinuity Guide, so I thought I'd address it. This point is simply that if Earth is abandoned, then there is no need for Styre to test humans anyway. In fact, I disagree; the script informs us that human colonies control "half the galaxy" and the Sontarans are planning a widespread invasion of the entire galaxy, not just Earth. Since this would obviously bring them into conflict with humanity, it makes sense of Styre's ghastly project and since Earth is abandoned it is makes a sensibly secluded location for his experiments.
These debatable issues aside, 'The Sontaran Experiment' is a well-plotted, well-placed and effective little story. The return of the Sontarans is more than welcome and Styre is an excellent villain, Kevin Lindsay once more donning a Sontaran costume to great effect. Whilst I prefer Linx's more closely fitting mask, Styre's is nonetheless impressive and Lindsay is superb as the Field-Major in every aspect. Although he is another Sontaran, Styre is a very different character from Linx; whereas Linx was ruthless and callous, he was an angel compared with the utterly sadistic Styre, whose pleasure in his work seems to extend beyond mere professionalism (from his point of view, he should probably have killed Sarah immediately, but decides to torture her to death instead). Lindsay very well conveys Styre's casual cruelty and also his brutality; the fight scene between Styre and the Doctor is rather good, despite Terry Walsh standing in for the injured Baker, with Styre lashing out with a machete with vicious rage.
The regulars are up to their usual standards, with highpoints including Harry's utter Fury at Styre's cruelty towards both the dehydrated Galsec colonist and the seemingly dead Sarah; until the Doctor stops him he is determined to go after Styre regardless of the danger. Another great moment is the first meeting between Styre and the Doctor, when Tom Baker delivers the line "you unspeakable abomination" with such conviction that he seems to genuinely loathe his opponent. It is perhaps not the easiest of insults to make sound convincing, but he manages it with ease.
Completing the ensemble, we have the Galsec colonists, and there isn't a bad performance amongst them. The decision to play them with South African accents is a good one, making a nice change from humans of the future speaking with an English accent. Their costumes are impressive as well, since they look convincingly worn and tatty, as they should do after days spent rough in the wilderness. Pete Rutherford is convincingly tormented as Roth, and Glyn Jones' performance is almost good enough to compensate the fact that he penned the dire 'The Space Museum'! Peter Walshe is impressively twitchy as the nervous Erak, and Donald Douglas completes the group as the treacherous Vural, playing the character like a natural.
Basically 'The Sontaran Experiment' is a brief but enjoyable story, and benefits from superb location work and solid direction (even Styre's robot, whilst suspiciously flimsy-looking, works adequately). It maintains the high quality of 'The Ark in Space' and nicely bridges the gap between that