For the most part, it's pretty accurate for me and I think members on here would agree.
The meter is not real time meaning it's not showing what youre getting at that exact moment, but an average of all your driving. And if you reset the trip A or trip B meter every time you fill up, it will be the average of that tank. So lets you drive 100 miles and the meter shows 22.X, that is what youre averaging and if you were to fill up at that moment, you would probably calculate ~22.X also.
Trip A and B both have a separate meter, so I track the current tank I am using with Trip A and you can get a sort of instant with Trip B. It takes about .3 mile before it will give you a reading though. I also found that the meter is pretty accurate to a couple tenths MPG of actual calculated gallons/miles.
The missing features you are looking for were deleted on US cars, I wish I knew why or if we can replace the clock to get them, would be nice. I suspect they did it to up sell the NAV package as it does all this and more, but like many people I too decided GPS and a trip computer wasn't worth another two grand.
I find the MPG meter fairly accurate, but it is a tad more optimistic than my method. I reset both trip meters every time I fill up and I calculate my MPG by taking miles driven on the tank, divided by the amount of gallons needed to fill up. The MPG calculator on the dash is usually optimistic by about 1mpg.
I did like MrEllusive and I found it to be top accurate, well I did all the calculations in my head while driving so mine is an aproximation but didi it for the first few months and the nagain in winter and it's fairly accurate