Polar S625x vs. Polar RS800cx. Is is worth to upgrade?

If you don't have any of them and wonder which to buy, go for the RS800cx, the S625x is on the way out of production, last summer I could not buy new cycling sensors.

Both these watches are built for persons doing multiple sports,  that is at least running and cycling. If you only do one of these sports, there are probably another watch that is less expensive as still fulfill your needs.

It is rather expensive to switch from S625x to RS800cx, especially if you have several sensors that you use, because you have to buy new ones. The old ones are not compatible. The new sensors use a different transmitter frequency. This leads to one of the improvements of the RS800cx:

Sensor technology improvements: The new sensors are less susceptible to electromagnetic interference from other equipment. Where I live there we have electric driven trains and several cycling paths along the railroads, this gives long periods of no sensor input. Another improvement is paring the sensors with the watch, so that the watch does not read input from other sensors when exercising with other people. I must say I have only rarely noticed such misreadings. The downside of the new transmitter frequency is that there are a lot of other equipment that can read the old Polar heart sensor input, that will not work with the new sensor. E.g. training studio equipment. This is only troublesome if you use such equipment to do physical fitness tests.

Pulse zone awareness: The RS800cx is fully aware of pulse zones, not just one predefined target zone, but every zone you have defined in the accompanying Protrainer 5 PC software. Thus you don't have to memorize the pulse limits for every zone. After the exercise the watch also lets you see how much time you spent in each zone, just as the PC software does.

Defining exercises: You can define exercises and store in watch. You can use the PC software for this. I have not studied this fully yet, but from what I read it looks like a great way to predefine you exercise settings so it is quick and easy to switch between various activities. It does also sounds very flexible, so maybe I don't have to bring my old Casio watch when running 20sec intervals. This brings me to another improvement:

Speaker volume: In my opinion the S625x have a too low speaker volume to hear it when you use it to control an exercise. This seems to be improved with the RS800cx.

Multisport: The S625x did not have support for doing a sport where you switch sensors during the activity, without stopping the logging and switch to a new sport and start a new one, which is not an option during a competition. The RS800cx have support for this, I have read but not tried. In any case since it has GPS support, you don't have to switch sensors.

GPS support: This truly make the watch capable for multiple outdoor sports, skiing, kayaking, etc. This may also save you from buying running and cycling sensors, especially if you have several bikes you switch between.
The downside of the GPS sensor is that it, as all GPS sensors, uses some current and has limited battery capacity. They say 15 hours for a Alkaline AA battery. Since is always annoying to get an empty battery during an exercise, you probably want to use rechargeable batteries that are always recharged. Which means you have to get into a certain routine.
While that is said, it is really fun to get the GPS track of the exercises. The integration with Google Earth is very good. The track is colored with the pulse zones, the data for intermediate and final times are included. So you can use Google Earth as an archive of your activities.
The GPS gets its signal rather quickly, it compares well with the newest Garmin GPS equipment, this will vary with the conditions. When I try this in less than optimal conditions, it gets ready in about 30 sec.

Display readability: As one gets older, the small characters on the display gets harder to read. The two uppermost lines of the RS800cx display has smaller characters. However it does have a zoom function that double the size of either of the these lines. So with a little careful preparation of the display setup, you don't have to read the small character during an exercise. With the zoom function, the display is easier to read than the S625x. This is an advantage especially in poor light conditions. I currently don't have any problem reading the display in normal clouded daylight conditions. I haven't tested it in sun yet, I live in the southwestern part of Norway,  only cloudy, misty, rainy and snowy conditions.

Design: I am no expert on design, but the watch does not look bulky as the S625x do, but it is still a big watch. They have put a big glass on the top of the watch. I wonder how that will work with scratches etc. The big glass is also slightly bent which gives a reflection from more angles.

It is also worth mention that the RS800cx can measure and store RR intervals. This gives the possibility to calculate heart rate variability. I have no clue what this can be used for though.

There are also many other improvements that I have not mentioned. This also goes for the Protrainer 5 PC software that replaces the Polar Precision Performance software. I think that both the watch and PC software has matured a lot. It looks as they have examined and listened a lot to customers and fixed some smaller issues.

So to answer the question "Is it worth to upgrade?", I would say it depends of how important these improvements are to you. GPS is a really fun feature, but I have used additional GPS devices for years. It does not add a lot to get this included into the watch. Sometimes, you want a GPS with map functionality, this can't this watch provide. As I said, it depends on your usage.