I went into the spa for one of the algae treatments while onboard the Mariner in April and while it was a neat experience, its overpriced and I felt super pressured to buy the products. Luckily, I had booked the treatment for the first evening we were onboard (boarded the ship at noon, went for my treatment at 5pm and made it to dinner with plenty of time to spare for second seating) and took advantage of the special pricing- half off. If you're looking to go to the spa, just wait till they offer the special pricing (usually the first night/day at sea and the last two days of the trip on a week long cruise) and make sure you book your appt. early on!
I also got my hair done in the salon on formal night. Call me biased as my sister is an exceptional stylist, but I wasn't too impressed with the salon staff. Still, it was a fun thing to do while onboard.
I agree though- I'll definitely be foregoing the spa this next time!