Nerve blocks are painless to do for the patient, quick acting, and makes the whole Juvederm experience pain-free.  My patients love it when I do nerve blocks for them, especially for injections around the lips.

Topical anesthetic creams are not helpful because the actual needle stick does not cause much pain, but rather it's the filling agent that hurts as it enters the dermis.  Also, with topical anesthetic creams, the patient has to wait 30 - 45 minutes for the topical cream to start working.

Local lidocaine injection into the treatment area (NOT nerve block) is not as helpful because it fills out the desired treatment area.  This makes it difficult to judge how much of the filling agent is actually needed. 

Nerve blocks are used in small volumes and often away from the treatment area so you don't have to worry about  distortion.

The only downside with the nerve block is that the area may stay numb for about an hour. 

Another trick I often use is mixing a very small amount of lidocaine/epi into the Juvederm -  this not only make it painless, patients are also less likely to bruise.