Hi... I am an ER RN, and I wouldn't work anywhere else... not because ER is so great, but because I can't hardly stand any other floor/specialty. ICU was ok, but I think ER is the best (for me anyway). Are you in nursing school now? If not, you can rest assured you'll get to do clinical rotations through almost all specialties and you can see for yourself what you like and what interests/challenges you.
ER ups: Fast paced, most of the time you get to get a new patient, then get rid of them (one way or another) fairly quickly... so not much of the medical/surgical bs that comes with long-term (relatively speaking) patients. It's challenging in that you must think quickly in many different situations and trouble-shoot all the time because you may see something you've never seen before even if you've worked for 20 years in the ER. You also may be granted some autonomy (depending on the size of the ER you work). Eg: I work in a rural ER where at night, I'm the only person in the ER, and the Doc sleeps upstairs until a patient comes in. So, I am able to order labs/xrays etc then get the results and call the doctor down.
ER downs: Gets repetative and monotonous sometimes (same ol headaches, drug seekers, medicaid abusers, etc that drive me crazy. It's tough to take care of someone when they are rude and they take for granted the fact that I'm the one paying their way to the ER (taxes, etc). It can also be hairy if you have a hard time dealing with death/abuse/other problems that can rip your heart out. Imagine working on a 4 year old that had it's neck ripped out by a pit bull. These situations are horrible and no one can get used to them, but they do happen, and some people just couldn't function at all and do their job well in this situation.
I hope this helps. This is what I've experienced so far. I got into nursing only to be a family nurse practitioner though, and I'm currently a student in a MSN/FNP program now.
Good luck in your decision making. Keep in mind you can do about anything with a nursing degree as far as advanced education is concerned (MD/DO, NP/PA, JD, PhD, PharmD, etc etc).