So what of the actual performance? Like every USB powered monitor we've seen the E2251FWU uses a TN panel so it doesn't have the excellent colour accuracy or wide viewing angles of IPS displays. In fairness to AOC horizontal viewing angles are excellent, but it suffers vertically with colours distorting noticeably when viewed from below. Again this is where the ability to tilt the monitor would have come in particularly useful.
Making up for this is the crispness of the display with text looking sharp and the 5ms response time just about holds up to most video content. Where we found it did suffer is gaming with first person shooters occasionally falling victim to ghosting. As such gamers should stay away and without native monitor adjustment so will photographers, but a life spent predominantly on the desktop and in the web browser won't cause it any problems. Ultimately for such a large screen powered entirely by USB the results are impressive.
Of course the important aspect to this sentence is 'by USB' because with a £123 RRP the E2251FWU is virtually the same price as entry level 22in IPS desktop monitors - such as AOC's own i2353Fh which has far superior performance, weighs the same and is better built. In this scenario is an additional power cable really such a burden?
Ultimately then it is hard to make a definitive judgement on the E2251FWU. Unquestionably there should be greater adjustability, particularly when on its stand, but as a large Full HD display powered entirely over USB its performance is admirable. The trouble is the E2251FWU fills a very niche space: users who want to regularly move around a 22in monitor, but find a superior monitor of the same weight and price too much to stomach because of a power cable. Until someone makes a 22in USB monitor with an IPS panel we'd suggest everyone else keep their money in their pocket.
Powering a 22in Full HD monitor entirely over USB is an impressive technical achievement, but it isn't very practical. The E2251FWU weighs and costs just as much as more impressive IPS desktop monitors, but its TN panel and low response time means its performance cannot compete. The E2251FWU will find a niche audience with those who need to carry around a large panel, but for the majority who are more likely to permanently moor it beside a desktop or laptop a more conventional display makes more sense.