I have been fortunate in using this airport as a transit on my flights back to India from USA many times. Usually I am in transit for about 1hr30 mts each way. Though being in transit you cannot see the whole airport but I have still found some good shops in the transit area.
If you are flying MAS or Singapore Airlines they stop in Taipei. The transit lounge is in the 1st floor (ground floor to some). Take the stairs upstairs and you will see quite a few good duty free stores. Some of them are quite big and have nice items. The prices seem to be on par with other duty free shops.
Next is the bathroom facilities. In this floor there are plenty of good bathrooms. In the transit hall itself there is only one bathroom and a vending machine. Thus go upstairs and you can avail really good bathrooms.
There are a few good restaurants also including a bar. I tried one where they had good coffee and Tea. Tea costs about NT$100 which is about $2 US. But they give a large cup and there is different types of milk and sugar that is available on the table.
In all it is a very nice transit facilility and you can really stretch your legs or have a drink and use the nice and comfortable bathrooms after a long flight.