I used Navionics
extensively in the Keys over the winter with my 28" draft
boat and spent a lot of time gunkholing in marginal waters. I would say generally the sonar charts
are very useful. The extra resolution helps in many places, and in some cases the sonar charts correct things that I know from experience to be wrong with the standard charts.
A huge word of caution has to be added though. I've seen a few places where the sonar charts are dangerously misleading. I frequently toggle between the sonar and standard charts and proceed very conservatively in places where there are discrepancies.
The other point is that the more you rely on charts to push the envelope, the more risk you're taking. Use multiple sources of data and don't depend solely on charts.