Apologies for joining this one late, but I think that there are more options out there than have been debated thus far. One option, of course, is to treat yourself to several field guides. Another is to leave the field guide at home & take a notebook instead (see thread on the "taboo" of using field guides).
If you've got the "Shell Guide" and don't intend to go abroad, then there really isn't a good utilitarian reason to get a new one; it will do nicely. However, I'm not convinced that the text is, as Jason opines, better than the Collins Guide and the latter certainly has much superior plates. However, if you're a relatively inexperienced birder then the non-British/European birds/rarities in the 'Collins' can be very confusing. This is where the Shell's relegation of rarities to the back pages is such a good idea. Having long and wistfully hoped for a European version of the Shell Guide, I now find myself hoping for 'Britain only' version of 'Collins' with rarities accommodated in the back. (I have a copy of the Shell Guide that includes birds from central & northern Europe birds, but it was only published in German & Dutch).
There's no doubt that the 'Collins Guide' is the best available if you're a keen twitcher who travels abroad, but if you don't fit this category ……...? If you stay in the UK and don't twitch then, as noted above, it has drawbacks. I'd look at the Kightley's "Pocket Guide to the Birds of Britain & NW Europe"; a good text, big maps, detailed UK distribution/population notes, more than adequate illustrations and all the birds you're likely to see in the UK without twitching. The relatively few non-UK birds are not so many as to confuse. It is also, as the name suggests, fairly pocketable.
The Macmillan Field Guide to Bird Identification by Harris, Tucker and Vincombe is, of course, brilliant so get it if you can. (However, don't confuse with the later Macmllan "Birder's Guide" with which it shares an artist and which is the same size. This book too is brilliant, but is aimed at the unless 'heavy duty birder" who visits the Middle East).
Rob Hume's "RSPB Birds of Britain and Europe" is by far the best photographic guide around, but in my opinion photos (even 'real' ones!) are still markedly inferior to the current crop of illustrated field guides. Don't just take my word for it, but ask most birders! However, if photos are your thing then this is the one to get. Personally I prefer the 'RSPB Handbook of British Birds'. It's not really designed as a field guide, but has good quality illustrations, good ID notes and masses of information.
I'd also recommend the flawed, but brilliant 'New Birdwatcher's Pocket Guide to Britain & Europe". The plates are superb with all plumages shown and birds at all manner of angles - as the are wont to appear in the field. Peter Hayman's style is not to everyone's taste being obsessionally detailed, but his illustrations alone challenge the Collins Guide for accuracy. It covers European species but has less therein to confuse (although it omits a few of the 'commoner' rarities). Text is good too. You'll just have to pencil in lengths (bizarrely omitted) and forgo the luxury of decent maps! It's greatest advantage is that you can pop it in a shirt pocket. Having been birding for many years I never carry a field guide in the UK (except when leading groups) on the basis that either I ought to be able to identify what I'm watching OR I ought to be taking notes & sketches. However, I do take this little guide along with me when abroad and some of the common birds are unfamiliar. John
