I can't say very much about customers' experiences, but our own limited testing with a T5 (which we just received a few days ago) suggests that PlecoDict works reasonably well on it, and certainly by the time we release the finished version we'll have done extensive testing / bug fixing. There are several known bugs with the T5 in general, however, so it may be a while before you can get a really solid experience on it.
Honestly I think you'd be better off with a T3 - it's more stable, it's $50 cheaper, same size display, higher-quality casing (metal versus the plastic of the T5), voice recorder (the T5 lacks one), vibrating alarm (again lacking in the T5), and though it has the same processor as the T5 it's faster in many cases because of the slowness of the T5's flash memory system. About the only advantage the T5 has over the T3 is the extra memory, and you can easily get a 256MB SD card for less than the price difference between the T3 and T5.
If you want WiFi, the T5's built-in flash memory is a little bit of an advantage since it leaves the SD slot open for a WiFi card, but at that point you might be happier with an alternate device like the Tapwave Zodiac (
http://www.tapwave.com/) - it has two SD slots, works with standard cheap SanDisk WiFi cards, and its 128 MB of built-in memory are all nice fast RAM instead of the slow flash of the T5. And it's still cheaper.
So in my opinion at least you should save your money and get something other than a T5.