People seem to have a variety of opinions about how various GPS receivers perform when used under tree cover, as can be seen in the sci.geo.satellite-nav newsgroup thread titled Tree Canopy.
While there are reports available about how GPS receivers perform under tree canopy, such as the US Forest Service GPS Receiver Testing Reports, most people just want a GPS receiver that will "work for them". A GPS receiver's technical specifications don't necessarily determine whether or not that GPS receiver will work well for you in your situation.
Knowing something about how the Global Positioning System works, and how your GPS receiver works, can help you make the best use of your GPS.
On March 31, 2004, I went on a hike on the Deeks Creek Bluffs trail, which is located north of Vancouver, off highway 99. Other than at the highway and a few clearings and viewpoints, the trail is in the forest, with what would be considered moderate to heavy tree cover. Near the start of the trail there are some steep cliffs next to the trail, and there are mountains in the area. I turned on my GPS receiver, a Garmin etrex Venture, at the car at the start of the hike. After it had obtained a position fix, I marked the "CAR" waypoint, and then carried the GPS on the right-hand shoulder strap of my backpack, in a cloth holder designed for an FRS radio. The holder was positioned below my shoulder, which meant that the GPS was in a position closer to vertical than horizontal. At a number of points along the trail, such as at viewpoints and clearings, I checked the GPS. If it had lost satellite reception, I regained reception before continuing. At one stop, the GPS had turned off, because the batteries had run down, so they were replaced with fresh batteries.
After lunch, I followed the same trail back to the car, again with the GPS on the right-hand shoulder strap of my backpack. As can be seen from the map image, unlike the first tracklog, the tracklog recorded after lunch didn't have any breaks, although the route was along the exact same trail. This was primarily because after lunch, I had moved the holder for the GPS further up on the backpack strap, almost on top of my shoulder, so the GPS was being held in a position that was close to horizontal. Another factor is that the signal reception possibilities may have been improved after lunch because the GPS was generally facing 'downhill', towards the ocean to the West.
One factor in GPS signal reception is the number of satellite signals that are available to be received, and where in the sky the signals are originating. In tree cover, it will usually be easier to receive signals from satellites that are "overhead", because the GPS receiver may be able to 'see' vertically up through the trees, but not horizontally through the forest. This is why you'll often get better reception in clearings, and especially large clearings, or ones on a slope that expose more of the sky. The hike took place on March 31, 2004, between 08:51AM and 14:27PM, PDT. The WGS84 location of the "CAR" waypoint is 49°30'15.26"N 123°15'02.52"W. Using a current almanac, the Mission Planning software shows which GPS satellite signals were theoretically available during the hike. The vertical grid lines on the map image are the North-South grid lines of a UTM grid, and the nearby mountains are on the Eastern side of the hike. Particularly in clearings, the GPS signal reception would have benefitted the most from the GPS satellites that were in the Western part of the sky during the hike.
The etrex Venture is designed to be used when held horizontally, in the left hand. It uses a patch antenna, which can be seen as the light-colored square under the 'globe logo' on the top of the Venture. While the Garmin etrex series of GPS receivers do not have connectors for an external antenna, they can be used with an external re-radiating antenna, such as the one from Pc-Mobile.
The above shows why it's important to understand, and consider, all the relevant factors before coming to conclusions, especially anecdotal ones, about whether or not particular antennas, or GPS receivers, perform well under tree cover. In this example, the main factor was how the GPS was used. There have been reports in the GPS newsgroups of people complaining about poor signal reception, but it often turns out they didn't know how to make the best use of their GPS receiver.