I have a mii band 2. This has a few more functions than the original mii band (watch, step count, heart rate, night time activity, target "buzz", idle alert, alarm). It is still cheap compared to other alternatives.
It was my choice because of the size (not a clunky bit of plastic stuck to your wrist at all times) and doesn't look tacky.
Whilst it is cheap, it is good quality. The battery lasts about a month. It is very reactive to steps. The phone app is much easier to read and navigate than others.
My partner bought a Garmin. He chose it so he can download his stats to Vitality (who do not recognise mii). But he spends many hours swearing at the app. However, it has the advantage of a watch battery which lasts a year rather than many bands with batteries that last less than a week: it doesn't matter how much functionality they have if the battery is always running low.