I bought this thermometer and found it really good. But I found the diary a bit of a faff so just used Fertility Friend instead - it's free and is such easy to use charting software. They also have a handy course in how to chart, so you can recognise pre-ov changes such as EWCM and not just look at the post-ov temp rise.
I bought a thermometer from ebay for £3.99 and have been using it since April (about a month after my MMC) Just got a BFP yesterday so it worked perfectly well for me! I would really recommend the fertility friend course. I learned so much. Good luck!