This is a great game :3
takes the original and upgrades on virtual everything.

new items
+ great new items, fun effects makes runs even more fun and sometimes challenging
+ They also worked on making items work together better, example (tammys head & brimstone tears) now you shoot brimstone out in all 8 direction, makes it more fun to find some awesome combos.

new enemys
+ great enemy variety, fun patterns to memories.
+ new awesome bosses, like the new bullet-hell boss.
+ ups the challenge of the original ten fold (I like this)

new room styles
+ new room sizes, like that i can fight monstro in a very narrow corridor.
+ new room traps and gimmicks like that.

Better performance allround
+ no lagging, even with a million things on screen (old one suffered from that)
+ Havn't had a single crash/freeze or bug with 100+ hours (thats a major win by todays standard)

Only 1 negative
- Even though the music in rebirth is good, it falls a bit short in comparison to the original.

I would say that this is a sure fire game, and i would buy rebirth over the original, feels and IS better in every aspect.
Ive had alot of fun with this, and would recommend it.

((Updates)) They added a bunch of more stuff to the game, add that to the + side.
((Additional praise)) I now own this game on several platforms, evidence of its value.