Affordable. Simple design. An assortment of colours and sizes to choose from.
Have used these on several bikes. Loud enough to let people on paths and pavements know you're there without them trying to do a 12 foot long jump when they realise you're there.
Also, the lever is much better than those with the flick springs and works well with thick winter gloves on. The mechanism also lasts forever unlike the springy version. Pity they don't do left and right versions though but you can always mount it upside down on the wrong side.
Excellent noisy wee bell, I bought 2