No complaints here. Call quality is great, internet speeds are acceptable. Overall its beats the pants off any previous phone I've owned. Only complaint I have is the inability to run applications off the SD Card and the low device memory.
I love my Sprint Curve. I have no complaints as well. I always have great service, internet speeds are great, quick sending SMS and emails. Also the Sprint Navigation provided by Telenav is awesome, and you can't forget about Sprint TV! Overall it's a great pone with good service, and despite what most people say about Sprint, I have had nothing but positive experiences with Sprint CS.
Almost EVERYTHING is really good except that the Sprint curve can't MMS...kind of lame but whatever, I can do the whole email/teleflip crap if I need to. Otherwise build, features, clarity and how user friendly it is, is GREAT. Specially coming from a Nextel 7520.
I agree with fraz and ballistix...I miss the threaded SMS and not being able to send pix to another phone is a little ridiculous. I've had the phone for a few weeks now and other than those two things I love it.