Hi Xue Sheng,
I've been to 2 workshops with William C.C. Chen. The workshops were 3+ days in length but I only attended 2 days where he focused on push hands and applications. His workshops are interesting and I did come away with material to work with when I practice. He is a very nice person as well. Easy to approach and talk to.
I originally started my T'ai Chi practice doing Ch'eng Man-Ching's 37 postures form and later learned a form derived from the 37 postures form and William C.C. Chen's 60 movements form (my old teacher liked sequences of movement from both so he created his own form). I moved to the Dong Famiy Style back in August 2005, taking lessons from Andy Holmes in Olympia WA (Andy has since taken a sabbatical from teaching to work on his own training and working with his teacher, Master Alex Dong). I like the Dong Family style better than the Ch'eng Man-Ching lineage style. My practice of the slow set brings greater feelings of energy in the movements than I got doing the derived 37 postures/60 movements form. Plus, some of the movements from the Ch'eng Man-Ching lineage style feels awkward (step back to drive away the monkey), but that's could just be me.
I'm not saying the Ch'eng Man-Ching style isn't good, just that for me I like the Dong Family style better.
Take care ...