I've got a 30GB Photo, a 4GB Mini, and a 1GB Shuffle, and an iPod Nano 4GB. 

The 30GB Photo:

The best screen of the bunch, and also the best battery life plus the best capacity, obviously. 

The 30GB Photo is good for keeping my entire collection of songs in one place, and to load photos when I need to.  Honestly, I don't really use the Photo feature, because I have a laptop that does it much better.  My laptop also has iTunes loaded, with my entire song library. 

If I needed to view photos, or access my entire song library at this point, I would just use my laptop.  If I had to choose today, I wouldn't choose the 30GB - it's too heavy, and there is a risk of hard drive corruption if shaken, dropped, etc.  It's also the most expensive.

The 4GB Mini:

Great for its time, but monochrome screen and price close to the Nano should put this out of consideration for all but those who absolutely insist on a pastel color on their music device!

The 1GB Shuffle:

The best looking thumb-drive I have seen yet for computer data storage, plus it can play the occassional song - but no LCD screen?  What was Apple thinking?  Get it only if you don't care to know what's playing, and don't want to be able to rate songs as you go.  In other words, don't get it!

The Nano:

The Nano, in my opinion, is the perfect iPod today.

With 4GB, I can carry about 400 songs at the best MP3 quality.  I only use 320kbps, 44.1KHz, normal (not joint) stereo.  I only use MP3 for portability, and not AAC.  400 songs gives me about 25 hours of play time, which is great for road trips pretty much anywhere!

In the car, the Nano is ultra-convenient as well.  I can actually just velcro it to the dash board - it's so light - rather than buy an expensive dock/cradle for it. 

The only possible fault with the nano is that the memory capacity is pretty small at 2 or 4GB.  But this is a matter of technology still being developed, and I'm sure that we'll see much larger capacities in the near future.  I'd also like to see Bluetooth built-in, so that it can play music through my car wirelessly - build this into the next edition Apple!

All in all, a great buy - you won't be disappointed.  Expect people who have not seen the Nano in person to stare at your Nano when you walk by!