I hired a car with Auriga about 15-20 times over the period 2-5 years ago. I never had a problem, and found the cars to be generally new, always in good condition and the damage waiver/insurance policy to be far more generous than at Hertz, Avis, etc. In our case they would always also waive the requirement for full/empty if you were only hiring for a couple of days (as you're unlikely to use the whole tank), but not if we were hiring for a week.
Car hire companies change, and a recent review suggested that they've had trouble financing the purchase of new cars, so their fleet is now older. I can't comment on whether that is true.
I always hired in Granada, where they relied quite a lot on repeat business, which is not so much the case in Malaga. Perhaps service was therefore always better in Granada.
Also bear in mind that some favourable/unfavourable reviews are almost certainly written by the companies themselves/competitors - look at the posting history of the person writing the review, so see whether they seem a credible source.
Overall, my impression is that Auriga have probably gone a bit downhill, but I'm not convinced they're worse than anyone else at Malaga airport, so personally I wouldn't change my booking at this stage. See what other people say...