My second RS400. The first lasted for ever, and whilst there are more modern units with the footpod you get all the stats you need and great battery life!! A great product for those that think charging every week is a faff!!! Also you can use the PC based local software to collect your stats - As I have 5 years of data I wanted to continue with the same data set.
If you're a runner or duathlete, you will need a lot of convincing to buy the much more expensive Polar RS400 SD compared to a Garmin Forerunner 305. After all, the RS400 doesn't have GPS, it doesn't have multi-sport capability or multi-sport heart zones, nor is it compatible with speed and cadence sensors for a bike. A separate GPS module can be used with the RS 400, but the maximum speed is 18 mph so it's no good for use on a bike. The heart rate readout does not include a heart rate reserve option. On the other hand, once the footpod is properly calibrated (calibration is three decimal places), I've found it to be as accurate as the Garmin 305 GPS. However, the S1 footpod is a shock if you're used to a dainty Nike+ sensor, as the S1 weighs 75 grams with batteries, it's 80mm long, 40mm wide, and about 24mm high, but despite it's weight and size it doesn't intrude. Although the RS400 and chest strap are coded against interference from other heart monitors, I'm not certain about the footpod; however, it hasn't caused a problem from other runners in close proximity (mainly passing me!).
The chest strap (Wear Link +) is compatible with Star-Trac and Life Fitness treadmills and cross-trainer equipment. Mine failed after 13 months - it was promptly replaced by Polar - so durability might be a problem. It can be a bit slow to dry if you use it every day; I have another Polar coded chest strap for those days.
The Polar ProTrainer 5 software is streets better than the Garmin or related software and, it's compatible with Windows 7 32 bit (I haven't tried it with 64 bit). Build 5.35.161 had an inverted pace scale, but this has been fixed in build 5.35.163 in December 2009. Different heart zones can be set in the software - for example, I used to use 180 bpm for Spin classes, gym, and cycling, and 187 bpm for treadmill and running outdoors, but the heart rate graph (known as the 'curve') needs to be checked to see that the right heart rate zones are used for the selected sport. Each of the zones can be set for any heart rate bpm, so heart rate reserve can be calculated and set for each zone.
I haven't used the training programs on either the Polar or the Garmin, but the Polar OwnOptimiser is generally a reasonable guide to my current state of training and recovery. Whilst the Polar OwnIndex has a very optimistic view of my VO2 max that bears no relation to the VO2 max predicted time and distance charts, the Polar Running Index is much better at predicting times for a variety of distances and pace.
The characters in the Polar displays seem to have a denser black than the Garmin. Even on overcast days, I can easily read pace and heart rate without glasses when the display is 'zoomed' to two rows. Although in the gym the display is not that clear.
The Start button requires one push to display heart rate or two pushes to record; it's easy to think that because the heart rate is displayed that it's recording, to find out later that only one push registered and it didn't record the session. The key here is to check that there is more than one row of figures displayed.
Slightly annoying things : the manual could be clearer in places, jacket sleeves can accidentally enable the stopwatch (but the buttons can be locked) and, being close to a radio mike in a Spin class can be interpreted as pace if the footpod isn't in use but it is enabled in the RS400.
If you're not on a tight budget and, good software analysis is important, the Polar RS400 SD is worth a look. For cycling, a separate cycle computer is a must. I would rate the RS400 SD at 4.5 stars if that were possible; bearing in mind that you also need the infra-red unit (unless your computer has an IR port), the RS400 SD would need to be quite a bit cheaper to rate 5 stars.