I got a 'First Response' one off ebay for less than £10 inc p&p, haven't used it yet as you have to start on the first day of your cycle. Have read the instructions and think it just does the same as FF when you enter your temps on there (except it beeps when you need to take your temp in the morning!). It builds up a database so it knows your cycle and can let you know when you're in your fertile period and when you're not.
I think it will be good as you can use it to prevent conception too and after my first baby I don't want to go on the pill or anything that could mess me up for TTC the next time (planning ahead quite a bit there

).
x
eta. it has a thermometer thing attached so you take your temp with it and it stores the data, don't think I made that clear.