I had one and loved it.
But, and it's a big big but...
I had it because I was struggling with the concept of pregnancy and to assist with bonding, not for reassurance.
If I was wanting reassurance I 100% wouldn't get a doppler. Midwives don't always find the baby, so you might struggle at times as well. Though TBF, I used them from around 12 weeks when it was tougher. I found out where baby liked to hang out it and ended up helping my midwife find her at my 16w appointment as she couldn't find her.
Though contrary to what some people say, I never found it hard to differentiate the different sounds - my pulse, baby and placenta. Once your know how they sound, that bit was easy enough
There were times though when I couldn't find baby for a day or so - I didn't panic, just tried again later and sometimes I found her, sometimes I didn't.
If you can fail to find baby, shrug, go out for dinner, forget about it and try the next day without worrying, crack on. If you'd spend the next 2 hours in years and wildly googling and wondering whether to phone the hospital, then don't.
Ps: that's not to mean you should ignore something odd that really doesn't seem right, but they'd a difference between well thought out concerns and anxiety.