Is Tall-Nut worth its price?

Tall-nuts have pros and cons to them. Personally, I use them instead of walnuts but your decision.

Pros:

• Take up less space than 2 wall-nuts, giving more defense area
• Recharges faster than 2 wall-nuts
• Pogo zombies can jump over the first one in a 2-wall-nut setup, but never tall-nuts.

Cons:

• As said by you, it costs 25 sun less to plant 2 wall-nuts
• 2 ladder zombies are needed to get through 2 wall-nuts, but only 1 for a tall-nut.

It's really your decision whether you use tall-nuts or not, however I advise you do.