I've been a tiel mom for a year now, so I guess I can participate! Keep in mind both of my boys were hand fed and well socialized as babies, so I got a good head start that someone working with a rescue or second hand bird may not get.
The good:
Friendly - this is probably the biggest one. My boys are so happy to just be with people, especially Miles. So far they have not been afraid of any of my guests... heck just yesterday they were climbing all over my mom after just meeting her 20 minutes prior. They really want to be with us, want to be involved in what we're doing... admittedly their persistence in being a part of EVERYTHING can get a bit annoying. And wherever one goes, the other has to come too! But you gotta love it.
Predictable - they are very easy to read thanks to their handy built in mood indicator (crest)... I have never been bitten by either of my tiels, because I can always tell what they want long before they have to bite to get the point across.
Stable - my guys are always chipper, always down to come out of the cage, very even tempered... of course, age and hormones could affect this down the line, but for now, they are consistent and reliable.
Easy to please - they're happy just to hang out and get some scritches or peck around on the carpet. Of course, that's not to say you don't need to engage them with foraging and training, but they are not going to destroy your house out of boredom.
Vocal - boys especially love to whistle and talk. You can teach them songs, phrases... Phoenix can say upwards of 10 phrases (not in context, of course) and it is absolutely adorable when he tells himself to step up on his perches or gives you an exaggerated MWAH! after a kiss.
The bad:
Dusty - This is something everyone's made aware of before they get a dust producing bird, but you don't really understand how bad it is until you actually have them. Prepared to have a thin layer of dust and feather dander on EVERYTHING... dusting daily is a must, as is an air purifier. Your lungs will thank you later on.
Vocal - This can be a bad thing, too... mostly for boys. They do not stop making noise! They are chattering, whistling, talking, and singing nearly all day. It's a pleasant noise, to me, but when they've been whistling nothing but "if you're happy and you know it" for the past 4 hours straight... it can start to wear you down. And good luck having a phone or Skype call if they're in the same room - their piercing tweets will drive almost everyone nuts.
Picky - They can be tough to feed. They love seeds... pellets and fresh foods are a constant struggle. Greens and carrots are about the only thing I can get my guys to eat on bad days.
The ugly:
Hormones - This is the only ugly I've experienced with my tiels so far. These guys can have some serious hormonal issues. Chronic egg laying in females, masturbating, regurgitating, screaming... Phoenix is going on 4 months of extremely hormonal behaviors and sometimes it is really difficult to deal with, but we hope that in time, he'll settle down.
All in all, they are AWESOME little guys. I think everyone should have a few.