I got one with DD and didn't find her until 15+3 and this one at 8+6.
I think they are great for reassurance. I've had a lot of anxiety with this one, and it's really helped keep my anxiety levels down without having to resort to anti-anxiety meds.
I think the cons are that you may worry if you don't find the heartbeat (most providers don't even try until 14+ weeks, sometimes even later). If it's going to make you worry more, I'd strongly advise against one.
I know this one is probably obvious, but it can be false reassurance. You can't use it diagnostically or to decide whether or not you should call your doctor/midwife. I don't know anyone who has, but just something to always throw out there. Though I do know people who have used their doppler and contacted their provider because of being worried, but if you're truly worried, always contact your provider rather than assuming things are okay because you heard the heartbeat.
Other than that, getting one is really a personal decision.