I bought a KORG AW-2U clip on tuner together with my KO'ALOHA soprano and it works simply amazing. It is worth to invest on this babies LOL. Find the attached link for review.
In my experience the clip on pickups are good enough to plug into a tuner with a jack socket, but I REALLY wouldn't dream of using one to amplify an instrument for a gig/open mic/pub session. Just not reliable or good enough. I've tried a couple.
This transducer pickup worked great for my classical guitars. It works best on a gloss finish. You may need to moisten the suction cup a little with your breath to make sure it adheres tightly.
There are many different types of pickups out there. It just depends on what sort of sound you want and what sort of environment you will be playing in. In the end if you want a good sounding pickup you have to be willing to spend the cash on one. Any style pickup in the $10 - $50 range is generally not going to sound very good. The pickups that I've found to sound the best usually require drilling a hole or having a wire coming out of your soundhole. MiSi and Pick up of the World are two that I know of that have had very good results but both will cost you $100+ for just the pickup.