I've said they're similar before.. because really when you try to tell people what the basic structure of guild wars is like, the only comparison is diablo.
As far as items and leveling though, It's very different from diablo. Diablo seems to have been made with PvE in mind, where as guild wars limits you on a bunch of stuff so Player vs Player battles are fairly balanced. So there are no uber-weapons, armor, etc.. And the only treasure hunting to be done is capturing elite/monster skills later in the game.
When you finish the game, there's not much else to do other than try to collect the skills you're missing, start a new character, try to collect the items needed for some of the unique armor (that doesn't have any better defense than normal armor), or play some guild-vs-guild.
If you're looking for a standard MMORPG/Diablo type system to where you can lose months and months trying to build up one character, guild wars may not be for you.
I like to think of Guild Wars as being more like a single-player game. It's an excellent game when you think of it that way. You can easily spend 50+ hours just doing the quests/etc, and you can always get some random online peple to help you do them when needed. When the game is over, there's still a bit of re-play value (much more than a typical single-player RPG).
It's definitely not a waste of money if you can get at least that much out of it. Its re-sale value is high, too. You can buy the game for around $30 used, then sell it a month later if you get tired of it for the same price - you can't beat free