I believe the Eris is "worth it". However I also agree with the Verizon dude. Let me explain.
The Eris is a nice little device. For me one of the fun things with this phone has been making it perform the way I'd like it to. There is a thread here on tricks and tips (I think it should have been a sticky) to making the Eris a very good smartphone. Also, I paid $50 13 months ago for my Eris, and got one for my wife too for free. At the time getting into a smartphone was usually at least a $200 proposition. So from a cost standpoint the Eris was well worth it. Remember this phone came out at the same time as the original Droid. And that brick was too much for me to haul around. And I have no use for a physical keyboard.
Now, why the Verizon dude was right. Since the Eris has come out there are so many more options. Droid, 2, X, Pro, HTC Incredible, not to mention Samsung and LG options. While I have enjoyed my year with my Eris, I am counting down the days until August when I can upgrade (probably to the forthcoming Droid Bionic). Also, 150MB for a smartphone is nothing. Anyone that thinks they can stay under that limit is fooling themselves. $30/month is 3 fewer trips to fast food a month. That is worth the unlimited data and the peace of mind that comes from knowing you can't get hammered for a heavy usage month.
I would suggest activating the Eris, having a ball with it, and when you are ready you can upgrade to something newer. Also, at $90 you are already in the green? Why? Because Verizon has a buy back plan, and currently is giving $132 for the Eris! Click here:
Verizon Trade In (Okay, the catch is that it is a Verizon gift card, but that is a healthy chunk towards getting one of the new smartphones.)