I currently use TWNGSM and I'm pretty happy with their services (it's the huge monthly bill that bites -- uh, but that's my fault). I'm glad that -- given a very competitive cellular phone market -- all the price plans are the same. It saves me the trouble to do too much research (I came to this thread cause I need to by an extra SIM card)
I got my TWNGSM subcription via my girlfriend, who got a two-for-one package from her workplace (corporate discounts). Our two year commitment to the phone plan finally ended last summer -- so I can now switch to a Pre-Paid plan and still keep my number. I was talked out of it when I realized how little I understood the monthly pricing plans.
Since my Mandarin is still pretty poor, I've pleasantly surprised at the level of English I can find at TWNGSM. Most of the time, the service people are very, very helpful. They aren't perfect, but I've come to overlook the occassional bump in the road: as Jeremy put it so well, "If the xiaojie tells you otherwise, talk to the next xiaojie"
Despite generally good coverage around Taipei (and underground) -- my entire apartment is a deadspot. This is my main gripe with TWNGSM -- so unless you live with me, you may not have this problem
Actually, it's gotta be the material in the walls, but same difference -- lots of times, I have to stretch over the railing at the back balcony-thingie to get a decent signal. When I'm home, it's hard to reach me by cellphone
A friend of mine almost convinced me to switch to KG telecom. He's happy with their Friends and Family type plan where calls to other KG users are free for the 1st 5 minutes. I feel like 80% of my voice calls are to one person -- so it would seem reasonable for me to join. Two things make me hesitate: (1) my huge phone bills are largely due to the large amount of travel I've done over the past 18 months, not because of voice calls within Taiwan; (2) I used KG several years ago, just before I switched to TWNGSM, and was pretty unhappy with their services -- although their networks and customer service have probably improved, they simply fell short in just about every category for me