To put whether it is 'worth it' in context.

Myself (with LapInfant) can get this pass.
MrLapLap, also visiting from the UK, cannot.

Current plans include this possibility:
MrLapLap acquires one portion of a Seishun 18 Kippu*
Departs from Ueno at something like 6:30am and arrives at Sendai just past lunch time = approx 6 hours and 5 train changes (much preferred by this traveller to a direct bus ride)
Returns to Tokyo with Shinkansen
Approximate cost of journey = 13,000yen

LapLap & LapInfant buy pass and meet MrLapLap in Sendai. Return with him later to Tokyo

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*Question I have is, can anyone recommend an actual outlet for obtaining a seishun 18 kippu portion? I won't have many opportunities or much time to go wandering about looking (and don't read kanji, so I'm not sure what the ticket outlets I read about actually look like)