Only you can say whether it is worth it.

First make a list of things that 2003 doesn't do that you need it to do, and
that you wished it did do/

Get a list of the new features of 2007 and see how many of the needed are
ticked, how many of the wished for are ticked, and then how many others that
you hadn't thought of but they sound good.

Then do the ticks warrant the expenditure?

Lotus 1-2-3 - hardly if you already have 2003.

I think Bob Phillips has offered a good way to do a quick comparison.  

I will add one caveat - if your work or anticipated application involves a
lot of charting/graphing with large numbers of datapoints, you may add a
point or two toward going with 2003.  As a review of several posts in these
forums will show, there are complaints of poor performance with such usage in
2007.  Microsoft has recently released a Hotfix: you must call to obtain it,
as it's "not thoroughly tested" - however no incident is charged to obtain it
and it does improve performance of charting considerably in the limited
testing I have done since obtaining that Hotfix.

To offset that you might consider that more than likely the user interface
introduced with 2007 is going to be around for a while.  It sounds like
you're deciding what to choose for your first spreadsheet application?  If
so, you'll probably be using it for years to come, with upgrades coming
later.  If things seem to come up as a 'tie' between 2003/2007, then I'd go
ahead and get 2007 and get on with the task of learning its user interface.  
Just be sure and mention that you're using Excel 2007 when requesting
assistance so people will know to instruct you properly on where the
functions needed in the user interface are.

You mentioned Lotus 1-2-3, and I haven't used it in years.  A good program,
no doubt, but not as widely used and for business use, Excel is probably
*the* standard, same for educational use.  One other alternative to look for
that is very Excel 2003-like would be CALC - the spreadsheet component of
OpenOffice, which is free for the price of download time.  The worksheet
functions are the same, but the macro language is not the same as Excel's.