Must've missed this one..I have two rena smartheaters and have had them over a year..as vw says sometimes the temps aren't calibrated well with the dial (I noticed this more with my hydor ironically). My hydor lasted just over a month before failure to operate for no apparent reason, so I went to renas and have been happy since. I have a 100w heating my 30gal which sits right next to an airy 10ft sliding glass door and the temp stays a steady 78-80 (Set at 79).
The only initial problem I had was that due to its design, some of my noctournal fish were going down inside the heat shaft. This did little more than make me nervous as the fish didn't seem to mind, but just in case I took it out and hot-glued a screen cut to fit over the top.
Also note that sometimes digital thermometers are poorly calibrated as well. Particularly the crappy ones they overprice at places like petsmart.
At any rate I wouldn't go back to hydor if you're disatisfied by the rena heater. The 3 products I bought from hydor all failed in a year or less. I think they were covered by warranty but its just not worth the hassle.