I think this thread is a great idea, myself. I don't have any personal info, but the growFAQ section dealing with pH/TDS meters and testing contains this nugget:

What are the pros and cons of buying a combo meter?
Contributed by 10k:

The combination type meters are real handy for the convenience of being able to take both readings simultaneously, or with a single touch of a button to switch between modes. The problem IMHO with combination meters is pH sensors like to be stored in a fertilizer solution, but TDS probes like to be stored in distilled water. Storing the pH probe in plain or distilled water will damage the ph membrane, so the combination probe needs to be stored in a fertilizer solution so as not to damage the pH portion, so the TDS probe ends up being "dirty" from salt buildup. A friend has already lost one expensive probe on his Hanna from this same problem, and will only purchase "single function" pH or TDS meters in the future.
Added on: Friday, March 2, 2007 Viewed: 178 times.



There's also mention of several brands of meters (and a table showing conversion factors between EC and PPM for various brands), but no reviews or anything like that. The growFAQ mostly seems to stay away from specific 'commercial' endorsements.

And in the RIU spirit of DIY, here's directions for your own PPM meter.




EC/TDS/PPM Meter