The indigo trisulfonate is quite accurate provided that the indigo is pure and fresh.  Dissolved ozone concentration can be underestimated by as much as 25% if old indigo is used.  The sampling is also critical, to ensure that ozone does not decay in the sampling lines..

For more information, refer to K. L. Rakness. 2003. "Ozone in 'Drinking Water Treatment, Process Design, Operation and Optimization", American Water Works Association publication.

Antonin,

As Barry points out, fresh is best; however, there are methods for adjusting the sensitivity coefficient of the indigo stock.  If the indigo is old or sourced from different suppliers then you should adjust the coefficient regardless as it can vary quite considerably.  Here are a couple references to get you going (in addition to Rakness 2003 which is a fantastic reference):

GORDON, G. & Bubnis, B., 2002. Residual ozone measurement: Indigo sensitivity coefficient adjustment. Ozone: Science & Engineering, 24(1), pp.17–28.

GORDON, G. et al., 2000. Using indigo absorbance to calculate the indigo sensitivity coefficient. Journal American Water Works Association, 92, pp.96–.

In our lab we calibrate the probes using the indigo method, so in terms of accuracy I would say they are as accurate (equally) as you are precise when preparing the indigo and collecting your samples :)  The benefit of probes is that you can track, control & record the process.