How to get to Barcelona cruise terminal and benefits of using hop-on hop-of

The most economical way is to take the Metro to the Drassanes stop (at the bottom end of the Ramblas), then cross to the south side of the traffic circle at the Columbus monument (yes, he's pointing southeast, not west) to catch the shuttle buses that run the length of the cruise ship pier.
The Bus Turistic hop-on hop-off bus only offers 2 day consecutive passes. But the cost, at 28 Euro, isn't much over their 22 Euro single day pass price. I've found the three routes they provide to be excellent.

We stayed on the Ramblas last year and took a Taxi right outside the hotel - if you have more than one carry on, it may be worth it to just get a cab. We also did the HoHo for our first day (helped with jet lag) and we were able to get off when we wanted and stay on when we were tired. We saw the whole city in one day and even my husband who prefers to walk admitted it was the best way to get our first overview of the city - we never would have seen this much in one day. And we got a discount coupon from our hotel and saved money on our tickets!

I think the above two replies provide the best options for getting to the cruise terminal.

As for pros & cons of the hop-on hop-off way of seeing the sights, my opinion is:

Pros:
Easy transportation between major sights.
Not required to figure out the metro system of a city you will only be in for two days.
If the weather's nice, it's a good way to see the scenery (waterfront & beautiful buildings).

Cons:
If you are going on "cruise ship day", the lines for some of these tours can be crazy long.
More expensive than the walking/metro/regular bus option.
Barcelona has great side streets (lots of shops and restaurants) that you would miss if you just took the bus to a site and then back on the bus to another.

For the record, I wasn't planning on using the hop-on hop-off bus system when I was there but after getting lost for half a day trying to find the Picasso museum and ending up walking to the Sagrada Familia (don't ask!), the hop-on hop-off bus became a much more attractive option. And I thoroughly enjoyed using it. I was there later in the fall so only 2 of the 3 lines were running, but was quite impressed.

4 of us from the cruise ship bought a multi ticket metro pass which we shared. We took the Metro to Sagrada Familia then back to the top of Los Ramblas and were able to walk down to the Columbus statue and then take the shuttle to the ship. Next day we took the shuttle back to Columbus and were able to walk eveywhere from there.We found the shuttle very expensive but ,as fit as we are,it really is too far to walk to the cruise terminal especially if you have baggage.

Steve, Norm, Donna, CP, and Margaret, thank you all for your responses. They are very helpful.