To be honest, there is nothing exceptional about N Tower in itself. What does make it exceptional is the experience around! So definitely put it into your 'must experience in Seoul' list.
The top of the tower is at almost 480m above sea level and sits in the center of Seoul which makes it a perfect viewing spot. Since the opening of Seoul Sky in Jamsil, N Tower is not the only cool viewing platform in Seoul anymore, however, in my opinion, N Tower still provides superior experience (and for less).
How to get the best out of your N Tower visit?
Start your visit 2 hours before the sunset. Get off from line 4 at Myeong-dong station exit 2 or 3. Walk up the hill through the 'Seoul Animation Street' to the cable car station.
If you want, you can use exit 4, follow the main Sogong-ro road and use the lift to the cable car station, however, following the Animation Street will be more interesting.
Take the cable car to the N tower (buy only one-way ticket) and enjoy some good time at the N Tower.
On the way back walk down the hill and enjoy some great night views of Seoul. The path is very well maintained, and there is some excellent viewing platform along the way. It is not a surprise that this is a good dating spot for young locals.
Cable car: Adult return ticket is KRW 8.000. One way ticket KRW 6.000. I always go for one way and walk down the hill. The atmosphere is great.
N Tower Observatory: KRW 10.000