I'll say this: I'd give it 4 1/2 if that were an option, merely because the place is so popular it's often hard to find a seat! But that's usually a good sign.
The coffee's excellent - no burned beans like Starbucks nor the watery sugary mess of Dunkin' Donuts. Just good, smooth coffee. The espresso bar covers the standards well, and adds some creative twists like the "Nutella Breve" (very sweet, but delicious as a dessert).
It's your typical indie coffee house - comfy, broken-in couches and mismatched silverware. But it does it exceedingly well. There's almost always some great, semi-obscure CD playing and the employees are friendly and skilled.
Also a selection of bagels, scones, etc etc - delivered fresh daily. I love the bagels - and you get to plaster on as much creamcheese as you like.
At night Tea Lounge serves a great selection of beers as well.
And oh yes, the tea... Well, there's about a ba-jillion different options, served loose in a personal teapot with cup on a cute little woodblock.
I've been coming here for about 4 years now and seen it get busier and busier. It's booming, and for good reason. There's even a second location about 10 blocks away. Tea Lounge is great. It's my first choice for coffee in the Slope - except when the line's too long...
Also - be aware that a group of moms/dads/nannies meet regularly during daytimes with their kids. I can't quite figure out the schedule - but it's stroller city when they're there. If you hate children, be forewarned. But if you like kids, there's a good chance you'd the chance to join in a singalong of your favorite childhood ditty.