I can see why there is a lively ongoing debate about the value of Manuel Antonio park, with or without guides because it is a mixed bag right now with half of the trails closed down for maintenance. Here is my three cents:
The one mile or so walk into Manuel Antonio beach from the entrance is indeed a little underwhelming. We went after lunch when there are fewer people, but it is still a wide dirt road with lots of foot and some car traffic.
We did see quite a bit of interesting wildlife, but only because we hired a guide through Manuel Antonio Expeditions. I think he knew in advance where a few sloths hang out, but also had amazing ability to spot stuff in the underbrush, particularly birds. Even then, you could usually only see them through his telescope, so I disagree with posters who suggest foregoing the guide and just wait till you see a huddle of people peering into the selva and join in. You might see a few things, but without the telescope, not a lot.
The MA beach itself is spectacular. It's a cove with rocky outcroppings on both sides. If we had more time in the area I would have taken one day to pack a picnic lunch, pay the $6 to park and the $7 entrance fee and make the mile walk, just to spend a few lazy hours there.
Our guide was Jason Solano Zamora 8723-1892 firstname.lastname@example.org. I recommend him enthusiastically for any kind of tour in Quepos/MA area. Good english. Friendly manner. Knowledgeable about plants and animals and a great eye for spotting them. I think he works on contract for MA Expeditions so you might be able to cut a better deal in advance by working with him directly, though I can't vouch for that.
He loves birds and spends his off time going on his own birding expeditions in Costa Rica. Can do all sorts of calls himself and has others programmed into this cell phone. We are not birders so I have nothing really to compare him with, but I suspect he might be a great birding guide if you are looking for one.