What should I look for when buying a smartphone handlebar mount? I am not interested in a specific product recommendation in answer to this question (like the existing iPhone question), but rather the key attributes to consider when buying a smartphone mount. Please be detailed and specific when describing each attribute and the pros and cons of different choices. Two example attributes:


Mount held by twist ties or screws?
Is the mount universal or model specific?


For each attribute please indicate whether there are trade-offs between choices or whether you always want to go with one choice.
I have owned 2 different smartphone mounts (Blackberry, iPhone). Before getting a mount, determine if you have a straight section of your handlebar that's at least an inch long, preferably 1.3 inches or more. Mounts with rigid plastic secured by screws will require that.

Key attributes:


Favor rigid molded plastic mounts secured by screws over the kinds secured by twist ties, as screws are more likely to keep the mount from slipping than twist ties.
Some mounts are made for a specific phone model, some made for a
family of phone models, and some are universal. My personal
experience has been that the universal or family types require
fiddling to get working right, such as inserting extra padding in
order to better secure the phone. Specific phone
model types typically require less fuss to hold the phone securely in place.
Many models are adjustable thanks to a ball and socket. This is a
good feature so that you can tilt the phone to face exactly the
direction you want.
Consider whether you want a two part model that has a handle bar mount portion and a
specific phone portion. This allows you to have mounts on multiple
bikes so you can switch the phone specific version between bikes.
That can allow you to use old mounts when you switch phones and/or
save money from not having to buy full two part kits. However, if you ride on very bumpy roads or off road, you need to be sure that the two parts lock together, as the smartphone portion may detach and fall when subjected to severe vibration (there are specific complaints about this with Arkon's two piece system).
Consider whether you'll be using your smartphone in rainy or very wet
conditions. If so, you'll need to be certain it's waterproof, yet
with a thin enough covering film so that the touchscreen still works.