Saying that the VIta and PSP are similar is ludicrous at best, they are extremely different and provide different games (Vita offers most of the PSP's digital library, too). Vita has many new features and can support superior games. It's like comparing PS2 to PS3 or original Xbox to 360. The features and libraries are completely different. (well, OK, not completely different since the Vita has some backwards compability, heh)
It all boils down to preference, however. Do you like handheld gaming (wether because of multitasking or because of portability)? Do you like the system's features? Do you like the game library (I suggest checking the digital library, since there's quite a lot of digital-only games on Vita and none are retail-only)?
The Vita has been pretty much worth it for me, partially because I have no PS3 and there's a few games that are on both PS3 and Vita only (such as Dragon's Crown, to name an example). It's also the best handheld system for proper indie games (not the trash that mobile platforms like Android and iOS usually get) like Hotline Miami and Spelunky and I do quite enjoy such indies games on a handheld, particularly since indie games are relatively cheap but still quality games. I quite enjoy its features, such as having two sticks (instead of the single analog "nub" of the PSP) and the touchscreen is a nice addition, but also provides some features like PSN interactions (trophies, friends, online MP, etc. All missing on PSP).
So, for me? Definitely worth it. But as I said, boils down to preference.