I visit this station most weekends;station staff are very friendly to spotters. However (freight wise)usually only see class 66's and the 08 shunter. Occasionally get something like a 37, but not that often.

had a good day there 14/9/11
no bother from the station staff
loads of spotters in attendance
toilets + cafe ok
quite a bit of freight
ews freightliner etc
plus usual first + virgin passenger workings
would definately go again

Two years on and things have changed a bit. Today the yard shunter is the only (almost) guaranteed sighting, BUT there may be others slouching around, usually EWS 66, occasionally EWS 60 as well. Overall, the years of much motive power not being gainfully employed seem to have gone which must be a good sign?

Didcot is a nice friendly station.

The best place for viewing would be the East end of platform 4/5 as this gives a clear view of the passing loop up to Oxford. The same end of platform 2/3 gives a similar view but with 4/5 between you and the loop. The advantage being with 2/3 that there's a couple of raised flower beds that can be used to sit on.

From either of these you will get a great view of all passing and through traffic (HST's, 165/6's, 180's, 220/1's, Sheds [Fred's passing and EWS into the yard] and the occasional tug), but you won't see the sheds coming up from Bristol/South Wales bringing coal to the power station. In my limited experience you can get up to 3 of these in an afternoon. However, the loco does run around and comes onto the station to do so. So either a walk or a pair of bino's will get you those.

Sometimes, especially if there are problems on the line south of Reading to Basingstoke the Fred's take the west loop - and bypass the station all together. If you've a good set of bino's you might be ok, but no chance to photo.

You will also have a few EWS sheds stabled, but they do so right by the station so can't be missed. There are two 08's in the yard but I've only seen 08676 working of late, and if you can't see this from the West end of the station it will almost certainly work by at some stage. The other 08 seems to be dead and is right up in the yard, you can see it from the footbridge but I doubt you will ge the number. Sometimes there's a shed stabled up by the depot building, you can see this from the footbridge and/or take a walk through the car park.

Toilets, cafe and shops all exist on the station and there's always a few other spotters here.

Update for Didcot station.
If arriving by car at the weekend it now costs £4-00.
Worse still if you park there during the week its £8-00.
Didcot is a good station with good clear viewing possible from all platforms.
Classes to see are 08,47,59,66,165,166,180,220,221 & HST.
Adjacent to platform 5 there are usually 3 or 4 EWS 66's stabled and in the sidings at the west end of the station there are usually 2 EWS 66's and an EWS livered 08.
There are toilets and eating facilities on the station too.
Well worth a visit but only drive there if you have too.

As an update for this location the current cost of the long term car park is £3-70 per 24 hours. No change is given and tickets are timed from time of purchase so you could do an evening one day and up to late afternoon the next on the same ticket. Food has been covered previously as has loco's. On a normal day on can expect around 70 movements in a two hour period. Most freight is 66 hauled although the odd surprise often appears, 43's of FGW are in abundance as are both types of Voyagers, 165's, 166,s and 180,s. Access to all platforms is free and the staff are sociable. An added bonus is of course the GR Railway Center is next door.

Nearly all the freight is 66 hauled. Anything else is not frequent

Didcot has always been a favourite of mine because of my liking for freight working. Most of the action can be viewed from the station. If, like me, you need to log wagons in the yard, you can see a good number of these from the footbridge over the mainline that leads to the car park. Many wagons can be seen from the car park too but this needs a bit more discretion for security reasons (the car park is covered by CCTV). Sometimes a loco can be seen inside the small shed next to Asda and this is also visible from the car park.
Food has already been mentioned. However I had to go into the town to get some cash and found a very good chippie on the main drag near to WH Smith (only if you can tear yourself away from all the action!)

How to reach Didcot

First Great Western services, from London and further north, Virgin also provide services

Locomotives that are normally seen at Didcot are as follows..
classes 37,47,57,60,66,66/5,67

The main photo point being the station, trains using the avoiding lines can easily be photographed from the station as well.

A good eating locatation is opposite the station , being a pub, which has great food.

There is a car park north of the station, which has a footbridge leading to it, so exit the station, turn right and follow the path until you come to the footbridge. Trains can be seen and photographed from here as well.
At the back of the car park can be seen Didcot depot, EWS locos can normally be seen here.

Also a variety of locomotives are stabled alongside the station, and can easily be photographed.