I find it's much easier to get to Rugby (by rail) these days since LM started operating hourly Euston / Crewe 'stoppers'. Unfortunately, the frequency of these useful services is much reduced in the evening.

'Pumpkin' on Platform 2 now closed - replaced by new cafe at road level accessed via main entry to the station, it looks O.K. but is a long way from the trains!

Make sure you get to Rugby when freights are plentiful, esp. late afternoon / evening.

If you drive to Rugby, don't use the expensive station car park. Use the Council CP on Craven Rd - a 2min walk away. This charges £2 for 5 hours (an afternoon's spotting). It's a bit rough and ready but my Fiesta has always been OK there. Walk down Murray Rd to the station and there's a shop on the corner of Abbey Rd. Get any provisions you need here - it will save you money later.
In May 2008 there was still plenty of work going on in the station area - plenty of guys in HVJs talking on their mobile phones. Since the infamous works of Christmas 2007 the tracks in the bay platforms have been removed. Thus there's no longer any locos stabled there posing for photographs. The best viewing spot is the north end of platform 4 where you get a great view of anything approaching from the north. If you have to buy food or drink, there's the Pumpkin Cafe on platform 2 - understocked and over priced.
Pendolinos of course account for 90% plus of the traffic. Play a game by writing all the fleet numbers down and then cross them off when you see them. In a daytime session you should see around 40 in a five hour bash. Class 350 EMUs do the local stuff - at least they're better looking than the 321s.
As for locos, if you are lucky you could get 20 in a 5 hour daytime session (on one occasion I saw only 9). There are 66s of all colours - for me the highlights are the DRS and GBRF varieties. Also regulars are Classes 90, 92, 86. Watch out for the Fastline Class 56s. There's usually a track machine around if you're into those.
In summary Rugby is still a good location - anything can crop up there. You just have to be able to put up with all those Pendolinos!
UPDATE - Nov'08
The middle platforms are currently closed to trains and the public. You can only use the 'new' platform 1, or the 'extra new' platforms 5 and 6. Thus viewing is thus not so good especially once the daylight goes. There's loads of guys in orange jackets around - some even working. Also some security guys (also in orange jackets) who give you suspicious looks.
UPDATE - May 2009
All the work is now finished and I must say that the station is now very smart (apart from no rubbish bins being provided!). The good news is that enthusiasts seem to be welcomed OK and the middle platforms are open again. Also there's a new stabling siding at the south end for locos(at my visit on 29/05 there were 2 class 573XX present). The only possible bad news is that the speed restrictions have been lifted - thus the Virgin trains go through at speed and it's a job to make out the numbers on Class 221, 390. Also photography is a bit difficult as there are some new fences. Rugby remains a good location though.

Rugby being near an OTP depot inevitably sees a relatively large number of these strange yellow machines compared to most other locations. They may not be everyones taste but they certainly add variety, which can only be a good thing in todays dull repetitive railway system.

Virgin,Virgin,Virgin Oh and Silverlink services to break up the monotany !
To the north of the station is a permanant way yard which sees regular loco activity along with stabled locos. Locos also stable around the station area.
There are a few freight trains to be seen during the day but a lot more from the evening onwards.
There is a buffet on the station and a newsagents round the corner. If you want Virgin then this is the place to be.