I have had the E71 for a while and largely found it disappointing. That said, it was my first smartphone, so I didn’t have much basis for comparison. More recently, I was issued a Blackberry Curve through work. On the basis of using both, I can say pretty clearly that the Curve is superior in most respects.
The E71 is decidedly clunky at multitasking. If you open more than one or two applications (and things like the call log and address book count), it can start grinding slowly and complaining of low memory. By contrast, the Curve seems happy to run a web browser, instant message program, and more with ease. Programs load much faster, and I have never had one crash on the Curve, while they crash often on the E71. My E71 has also been plagued by software bugs ranging from the annoying to the truly infuriating.
Voice quality is comparable between the two, and not especially good in either case. Reception is comparable in both, as are web browsing speeds.
The Curve web browser seems superior both to Nokia’s built-in browser and to the copy of Opera I installed on the E71. Neither really provides an ideal web experience, however. Both give the feeling of accessing websites through a little window in a piece of paper that you need to move around vertically and horizontally. Both also have trouble with some fairly basic web elements, such as logging into content management systems like WordPress.
I cannot comment on third party applications for the Curve, because I have not installed any. Generally, I have found those I had added to the E71 disappointing. A couple of notable exceptions are Google Maps and GMail. The built-in GTalk client on the Curve is rather good. One significant limitation of the Curve is the lack of GPS, which would actually make it much less useful as a primary phone.
Both phones have cameras that are too terrible for any serious use.
Aesthetically, the E71 wins hands down. It feels sleek and solid, whereas the Curve feels chunky and a bit soft. The E71 also looks a lot better, with nice differentiation of colour, the steel back and silver highlights. The Curve is a generic black rubber slab. The keyboard on the E71 is also distinctly better, even though it is a significantly smaller phone. The shape of the keys on the E71 make it easier to type quickly and accurately, while I find those on the Curve awkwardly sized, shaped, and spaced.
In an ideal world, I would put the guts and software from the Blackberry Curve into the body of the E71, with a keyboard and GPS transplant from the Nokia to the Blackberry offering. Given the choice between buying one or the other now, for personal use, I would probably opt for the Blackberry. It falls down on aesthetics and GPS functionality, but seems to be superior in most ways. Regarding my keyboard complaints, it also seems possible that if I had started off with the Curve, I would prefer it now.