Wish I could give you an unbiased opinion, but as I’m responsible for TaiwanCell I can only encourage you to give it a try.
We don’t have a lot of customers, because I don’t actively promote it. I spend most of my time reminding people to pay their bills. (They pay eventually, because we modified the contract to allow for legal action in your home country if necessary, but for some folks it doesn’t seem to be a priority to honour their debts.)
To be honest, once you get an ARC there’s no great advantage of our service over any other. Everybody adjusts their pricing from time to time, but generally speaking they all work out about the same if you pick the right plan.
The biggest advantage we offer is the fact that you don’t have to wait until you get your ARC. We can sign you up the day you arrive, which means that you’re contactable by potential employers. It’s hard to get an ARC without a job, hard to get a job without a phone.
We’re basically just resellers for Taiwan Mobile Company, one of the big companies. Technically the service is fine.
Hope that helps, and don’t forget you can always email, PM, or post here if you have questions.