I've used them before as well. I think they are a good idea if you are doing any serious training or any sort of competition. I sold the last one I had when I took a hiatus from IDPA. It was just sitting there and since I was almost always with someone that had one I figured I didn't need it. This one or a similar older model is what I had. I like it pretty well. Worked great and was compact. http://www.cedhk.com/shop/products/CED7000-Shot%2dActivated-Timer.html
I use this one... https://www.amazon.com/Competition-...F8&qid=1480876178&sr=8-3&keywords=shot+timers I highly advise using shot timers in a large portion of your training. Not everything needs to be timed. However, you do need a base line to improve. Shot timers help improve your speed and are a great way to compete with your range partner.
I am sure I will feel this way as soon as I start using it It will be for training, I have no interest in competition yet. My main use will be just that to put myself under a little stress and know weather I am improving or not. The first task will be to improve my draw and getting the first shot on target. I feel like I am doing it pretty fast but want to know for sure.
I've got the CED700. The display **** out on me within the first few months. They replaced it right away, so I cant complain about the customer service. I am still not a fan of the display though, it can be hard to read. At some point I will be selling it for something else....
I have never had any luck with those shot time apps. They are just not consistent. They are great for dry fire practice though.