I have that attachment for my Profi-Six and it is bulky, but it can be packed away and used only when required.
As for it's accuracy, if it's used correctly then it is as accurate as any other spot meter.
The really good part is that you have a very nice meter which can be converted to a 1, 5 or 10 degree meter with the flipping of the small lever you see on the side of the picture.
When in either the 5 or 10 degree mode, a red 5 or 10 lights up in the overhead position to alert you to the fact you are not measuring 1 degree.
You have a dioptre adjustment so you can look through without glasses and see things clearly.
I have found that the accuracy, when compared alongside any other 1 degree meter, is comparable. The ability to actually take different angles in the one meter at the flick of a switch is a distinct advantage over some other meters.
The viewfinder is basic and you do have to take the meter away from you eyes to null it, then work out your various exposure values from the permutations the dial gives you. But, if you are using a spot meter, speed isn't really what it's all about.
I love my Profi-Six and I have 5 of the 10 possible attachments.
I bought mine secondhand for $180.00 Australian (AUD) about 18 years ago. About 2 years ago I saw one going for $120.00 AUD and just very recently one was sold for $220 AUD (approximately)
I wholeheartedly agree that having one meter for everything means one less measuring conflict.
Mick.