I initially switched to symbian with 7610 from 6610. Once I got used to the slight drop in responsiveness, there was no looking back. When my 7610 dropped dead, I bought an e50, simply because of my huge contact database, which restored very nicely into the e50. v3 and active standby is also very nice and finally allowed me to ditch my Palm as well.
Now your specific questions:
RF/reception: Very good
Call quality: Very good, clarity is much better than 7610, except for intermittent popping noises.
Speakerphone: Serves the purpose (while driving I am often on speakerphone)
Every phone is always a compromise actually, just like a car. To sum up the e50, it is a very good overall VFM in a slim small package with good battery life. By removing the plastic spacer on the metal back cover you can also use the BL-6C - which will give you even better battery life.
It is pretty much fast, much faster than the 7610 (except for contacts - but then that may be due to voice tags [they actually work if you say them the way they are played back] and large database.
The problems you should know (and how I dealt with them):
1. The occasional popping sound (endure it - not too difficult as otherwise voice clarity is very good).
2. Earpiece heating up (reduce the volume 2 notches below maximum)
3. Occasional speakerphone crash - when pressing the green key to answer a call, it directly goes to speakerphone and after completing the call, hangs. (use anykey answer enabled and also 4 below)
4. Poor battery life (ensure no application is running in the background - I uninstalled the autokeylock and ensure contacts/log/sms, etc are all exited periodically. This has doubled battery life from 1.5 days to 3 days of moderate to heavy use)
5. Since it is a high res 240x320 screen in only 1.8-2", the fonts become too small. (My eyesight is ok and otherwise is quite usable if I stick to the bold headings, instead of reading the fine print! Also you cannot have a small phone and big screen, then the keys will be too small)
I felt the N73 almost double the cost for very little gain (I have no use for the camera). Hope the above helps.