I have been using Nextel for ~3 years in the Chicagoland area and have
had VERY GOOD luck with it.  Not 100% all the time but close enough.

My usage is mostly from the loop to the WI border within 15 miles west
of the lake @~800 min/month cellular and ~500 min/month DC.  Even at
peek times calls go through and DC is fine.  You will see some "User
Not Available CO" on DC at peek times but it never lasts for more than
5 seconds or so.

Call quality is great, very few drops, and I can usually stay
connected on a data call from Grayslake to the loop on the Metra with
1-2 drops normally around Forest Glen.

As far has hardware I have only used the i1000+ and the i90c.  The
i90c is a great little phone that can really take a good wack or two
or ten.  Many drops with no problems.  Battery life is what it is with
all iDEN phones; poor.  Just charge each night and you are fine for
2-3 hours of talking during the day but don't expect 5 day stand-by
times like you get with other OTA technoliges.

Finally, if you have a need for WAP service Nextel's is very good.
They do a great WAP 2-way SMS like service and Outlook/Notes/POP3
email.  It costs extra but is good if you need it.

Let me know if you have any other questions.

-D