ESR meter: Are in-circuit readings accurate?
It's quite useful for repair purposes. The voltages are low enough that semiconductors don't come into play. When there are parallel e-caps, you should be comparing a total ESR of the parallel combination, but you're really looking for gross differences not even the last 2:1. The typical situation is an older piece of electronics where the electrolyte dries out, causing the capacitance to change a bit, but mostly the ESR to increase, so you get mains ripple or SMPS ripple, which eventually causes the thing to stop working adequately. E-caps do eventually wear out, and replacing them can often restore an older item. Especially the caps in the power supply- because they usually have less margin and because they tend to be in a hotter area and have more self-heating (due to ripple current and ESR-- so the higher ESR gets, the more self-heating occurs).  

The other factor affecting usability is that a lot of recent electronics uses low-Z caps, so what's excellent for a standard e-cap is a bulging mess on a low-Z cap. You may have to look up the part number or guess from the schematic or circuit configuration (something like 1000uF/6.3V on a motherboard is just about guaranteed to be low-Z).  

I wish I had one of those, there's a few times it would come in handy, but I don't do enough repair work to justify it. Usually I just tack a known-good cap across the suspect one and see if it makes a difference!