We just had our Illuminations cruise last week,

and here are my thoughts:
-- Birthday decorations cost a bit extra and were worth it -- nice banner on the side of the boat, as well as bunches of balloons tied to the sides of the canopy (a little bummed we didn't think to ask to take the balloons along after, but the banner looks too nice for them to give away)
-- We had a party of 11 (6 adults and 5 kids) and had absolutely no trouble fitting all of us. Even though when I made the reservation they told me the max was 10 people, period.
-- Wally was our captain, he only works on Monday nights and he was great!
-- We tipped $40, and I think my brotherinlaw also tipped something too.
-- My FIL was thrilled with his birthday surprise, and ...
-- We had a WONDERFUL view of Illuminations!!
While I don't think we'd spend the money on this cruise every trip to the exclusion of trying other fun things at WDW, I'd definitely do it again in the future -- especially if it were just my dh & kids and we could split the boat with another family.
Have a magical day!