I've been swimming with the Poolmate for about a month now and on the whole I'm very happy with it. My swimming has improved significantly, partly because I've been able to accurately track my times, speeds and stroke efficiency but probably more because I'm not having to count my laps which leaves me free to concentrate on maintaining good form.
The Poolmate isn't without its quirks, as many reviewers have mentioned, but I've been able to minimise or work around all of them quite easily. I shouldn't need to say that you need to have the Poolmate tight on your wrist. Not so tight that it's uncomfortable, but the watch shouldn't move around on your wrist during your stroke. The most significant issue I've encountered is the Poolmate's tendency to 'drop' laps. Basically this happens when the watch fails to detect that you've finished one length and started another. It's a good idea to periodically check that the watch has an even or odd lap count at the right end of the pool. If it's wrong, then that means it's dropped a lap so just carry on and add an extra lap to what the watch says. All your figures will be slightly out, but your time will be correct which for me is the most important.
Chances are you can easily minimise the likelihood of the Poolmate dropping laps by changing the arm you wear it on, though. The watch seems to detect the end of one lap and the start of another by registering a stop in motion, followed by a pause as you kick off and glide to start the next lap. When I started with the Poolmate, I wore it on my left wrist, like I would a normal watch, but soon realised that because I take my first stroke with the left arm, I was shortening the amount of time the Poolmate had to register that 'glide' phase. By wearing it on the right wrist, it gets an extra half-second or so and now it almost never misses a lap.
A few minor technical hiccups shouldn't put you off this device though, as it's a great training aid and probably the next best thing to having a swimming coach. It's an innovative device and there's nothing out there yet that can do what it does, so a few minor limitations are to be expected.
That said, there are a couple of things that prevent me from giving the Poolmate five stars. Firstly, it just seems to cost a bit too much for what it is. It's essentially a waterproof digital watch with a motion sensor and some simple software and I think Swimovate are being a bit cheeky charging £70 for it. Secondly, the Poolmate Pro option costs twice as much for the same watch with a microUSB port and a bit more basic software. Thirdly, the battery is only expected to last for one year, at which point you're supposed to send it back to Swimovate to "ensure it remains watertight", the cost of which, judging by the rest of their pricing, will likely be a gross inflation of what you'd be charged at your local watch repairer (who would almost certainly do just as good a job).
Bottom line, if you're a keen swimmer and want to improve then the Poolmate is very effective and worth what it costs. If you're more casual about your swimming though, you should have a think about how much you want to spend on the Poolmate and its replacement batteries, and be sure you're going to get enough use out of it to justify the price.