I like George's response!
O-sensei said "Aikido is the completion of all religions" which is quite interesting.
"Spiritual" can mean things like religious or things like "polishing the spirit" or self-improvement like masakatsu agatsu.
That aikido is strongly spiritual is beyond question, although it can be practiced in a merely physical, technical way, which I think is sort of a "junk food" aikido personally, but most of the takes on it I suspect depend on the particular sensei, or the student who does a lot of reading.
I just finished reading an Aikido Journal (1999) transcription of an O-sensei lecture to the Byakko Shinkokai - it'll sprain your brain. One main message is that spiritually aikido allows us to "connect" or attach to the spirit of the universe (God).
Personally, I think that since effective practice requires being sensitive to where uke is in unfolding an attack, sensing ki direction, etc., that all of this promotes more sensitivity to others and their present feelings in the "off-the-mat" world. That increased sensitivity should result in more sensitive responses and actions, thus better ethics, thus better spirit development.