I wouldn't base myself in Saltash out of choice if I were working in Derriford - the traffic can be dreadful in/out of Plymouth. Same goes for Torpoint except that doesn't even have the benefit of being particularly nice.
There are plenty of nice areas far closer - Derriford itself is good as mentioned, Roborough/Woolwell fine too. Tamerton Foliot could be worth a look, has a nice villagey feel. Then there's Plympton which has plenty of good schools.
Personally I prefer living more centrally, and the naice areas of Plymouth itself would be Stoke, Mannamead/Hartley, and Peverell. Leafy with good schools. Areas to avoid within Plymouth would be Devonport, North Prospect and Stonehouse - these are the more impoverished parts if you want to avoid drugs and crime! Mutley, Greenbank and Lipson are the studenty areas so again avoid those if you want a quiet life.
What is Plymouth like? It's a mixed bag really. The city centre is pretty souless due to being rebuilt after the war, and is still lacking shops that you find elsewhere - no John Lewis for example. However the Hoe and the Barbican, walking distance from town, are lovely. The Royal William Yard has been regenerated and has lots of good restaurants. We have a very good theatre.
Primary schools on the whole are better than average, and there are 3 excellent grammar schools. Plenty in the vicinity for children - close to the coast and the moors.
The pace of life is pretty slow and crime rates below average. Job opportunities are not great though.