I'm going to add a few bits which mostly repeat/confirm what others have said in various posts/threads, but hey, all in one place
I made two J redemptions a few days ago, reasonable value TPE-YVR and then YVR-TPE-MEL. It's really hard to avoid the extremely long daytime layover in TPE because of the waves of longhaul landings (morning) and departures (late evening). If I had been able to position myself better in North America, I'd have gone for LAX-TPE-MEL as it's one of the few short-transit connections. Note that QF might not show the correct final miles until the booking process begins (it showed 175k approx for the longer redemption when searching but then reduced it to the correct 159k ).
Nonetheless, while a stop in TPE is long, Taipei is easily accessible (once the MRT starts! ... arriving from Australia at about 4am is not the best way to start (did that earlier this year, it's 2.5hrs until the MRT opens) and an easy place to explore (though preferably not too actively in hot and sticky summer, which can be April to November if you're unlucky
). You could even spend the day doing a modest hike in various nearby hills/mountains.
The T2 basement CI lounge is rather standard ... small food/beverage selection (the fresh beef noodle soup and a few other items are the modest highlight), not particularly great atmosphere but not terrible (it feels more pleasant than the really packed parts of the new SilverKris SQ lounges at Changi). There are showers. (* See end note about showers)
In the air, J and PE food tends to be fairly good, especially on the non-Western side of the menu. Service is friendly but not very proactive. The cabin aesthetic is really very nice, and seat comfort is excellent as long as you aren't very tall I think. The entertainment range is pretty poor (in all languages), and if you like watching animated movies as a nice way to zone-out in-flight, they're only in low-res and look terrible (and it has been like that since at least 2018). Given I have three longish flights with CI, I'll be taking additional entertainment!
My booking didn't go totally smoothly: one redemption ticketed within minutes, while the longer had done nothing after five hours, so as it was the most important part of my trip, I called QF at midnight and got through to South Africa and they claimed the credit card payment for the YVR-TPE-MEL redemption had been declined (it hadn't, the charge was clearly pending in my account, just like the other one that had been ticketed). There was a bit of discussion about this, and eventually some sort of "manual resubmission" was done and I was told to wait 48 hours and call back if necessary. Thankfully the ticket for that arrived about 15 hours later.
Note that I was also told that the auto-cancellation window for unticketed awards on CI was 10 days.
So far it has been impossible to access the bookings for seat selection purposes on the China Airlines website (under Manage>Seats) but as noted by others, access via the app works as long as you include first and middle name (if applicable) together (sometimes without space) in the Given Name field. I have previously been able to use the website as well for other QF redemptions on CI, so we'll see...
(*Showers at T2: 1. in the lounge if you have access, you register at the front desk, and then they come looking for you downstairs when one of the few cubicles becomes available. 2. There are two free shower cubicles in the airside baggage arrivals area, on the far lefthand side. I have no idea how much use they get. 3. There are free showers in airside departures 4F and landside 5F. I've used the former and there was no functional queuing system so it can be a bit irritating (plus I pressed a random button in the shower and then only had cold water therafter
).)
. 
