i love clipless pedals and have them on 3 (MTB fully, MTB commuter, road bike) of my 4 bikes - not on my old old MTB i use when i park somewhere i don't want to leave one of my expensive bikes...

for XC clipless are awesome. for downhill I personally need my pedals otherwise i destroy my shins...

don't know much about trials, but most the trials riders i have seen use various stuff based on pwersonal preference - power straps or wide platforms or clipless(although always also with the wide platforms).

i think unless you're doing huge freeride 6ft+ jumps or you are into BMX style tricks - which i personally think are cool but i can't do - then clipless should help your riding.

clipless pedals are more efficient so you get more power. also on bumps or technical sections the bike stays with you. i can bunnyhop higher with clipless pedals b/c you can pull with the feet and then also do stuff like hop the rear to the side and do track-stands easier... for me in rock gardens or technical stuff i MUST be clipped in or i'm worthless - i can't bunnyhop well or do a trackstand or hop or whatever well w/o my pedals --- probably the only problem i've had is practicing wheel-riding and then flipping over backwards while clipped-in - ouch! so always keep your hand on the back-brake to prevent this...

the 1st few hours are tough - even harder if you're used to straps and pulling back instead of twisting... but you should leanr it fast.

once you learn to get in and out quickly they are almost never a problem - in a few of my really bad crashes i didn't unclip before i lost control but the cleat alway turns rather than my foot breaking or something...

practice until you are really condifident - unclipping comes pretty fast and then you need to practice clipping in - now after a few years and a few thousand miles of practice i can clip in starting a drop on a downhill or uphill on near 20% grade...

if you want to be a good rider and are intersted in 'normal' stuff like trails and XC and some downhill and not specialized stuff like trials and BMX then i strongly recommend going clipless.