First off, I don’t currently own a field monitor. This is primarily because they cost money, but I’ve also been waiting for Tramm to get 1080i HDMI-out enabled during recording on the 5D (which may or may not make it into the next release). Currently, as soon as recording starts on the 5D, the output resolution drops to 480p, which makes an external monitor much less useful for focusing. This change in resolutions also introduces a number of issues with monitor switching; every monitor is different, but many monitors take a few seconds to switch modes, and repeated hundreds of times a day, this can be an exercise in frustration. The 7D has its own issues as well, and presumably the T2i introduces a host of new (but different!) problems. Despite these issues, however, I’m currently planning on buying the SmallHD DP-SLR, which at 5.6″ is a bit smaller than many monitors but large enough to provide for more precise focus monitoring while still being able to mount it to a handheld rig (with an articulating arm, which will allow for more angles). It’s very high resolution, has upgradeable firmware, and should work great on whatever camera I shoot with next (a RED, for example).
Until then, to digest what’s currently being said on the forums:
For $230 the Lilliput 669 is a great cheap option. A monitor at this price point isn’t going to have a lot of adjustment options and won’t offer the most accurate color reproduction, but again, if you spent $800 on a DSLR then you probably don’t want to spend $1,000 or more on a field monitor (some would disagree). The Lilliput has a 7″ screen and is 800×480 native resolution; there’s a lot of good info in this thread at Cinema5d. Note for the Lilliput you will need a few accessories (miniHDMI to HDMI adapter, batteries, etc.) to get properly connect and mount the monitor; I haven’t tested it myself but there are some forum tips in here as well as more details on battery options.
For $800 the SmallHD DP1 is a very popular choice; it has a higher 1024×768 resolution for what approximates to 720p resolution. Similarly to the LCDVF, the guys at the North Carolina-based SmallHD bootstrapped their own 10″ LCD monitor and are now selling them at volume, and the general consensus on the forums is the SmallHD can’t be beat at the $800 price point. I look forward to shooting with their smaller model; after all I have to help my fellow Carolinians raise up.
There are plenty of more expensive monitors on the market from manufacturers like Marshall, Ikan, Manhattan, and Panasonic, but for DSLR use I’ll move on; if you’re spending $2,000 on a field monitor, you’ll probably want to do your own research rather than take my word for it. If I do come across any of these other monitors, I’ll add my thoughts. In the meantime, Cinema5D has published a four monitor review, comparing the smallHD DP-1, ikan V8000HDMI, Marshall V-LCD70P-HDMI, and ikan V5600. There were also some new monitors demonstrated at NAB.