Sometimes it really does pay to be frugal, in a hobby that not only requires a considerable investment of time but also slurps down cold hard cash like the world is ending. Since starting out in this hobby I have sunk enough money into astronomy to buy Wayne Rooney for 3 and a half minutes, while there's no denying the benefits of owning a potato shaped prima donna even for 210 seconds I still feel my monies are better invested in the stars.
You see the primary function of a Barlow lens is to multiply any single eyepiece it's attached to by a certain number (in this case the image will be magnified 3 times). This serves the instantly appealing function of effectively doubling your eyepiece collection at a fraction of the cost. For example an eyepiece giving you 120x magnification will instantly yield a magnification of 360x by simply placing the barlow into your focuser tube before finally attaching the eyepiece to it.
This makes the barlow a great piece of equipment to own providing you get the right one, remember you can only see as well as the lowest link in the chain, it's no good having an exquisitely good telescope and a fabulous eyepiece if you use them in conjunction with a shoddy barlow, you'll end up with sub standard views.
It's also important to note that barlows are not for everyone as we'll go into, if you can, try and get a look through one before you buy, most reputable telescope emporiums will let you try in the store before you buy and failing that you could join up with your local astronomical society who would be glad to show you their equipment.
Bare in mind that it is useless to over magnify, your telescope will have a natural limit to useful magnification beyond which images will become washed out husks of their former selves. To that end it may be best to buy your barlow first and then pick eyepieces that will work best both barlowed and unbarlowed. A good example of this, when I was first starting out I rushed in hastily as I was of a penchant to do, I bought loads of Eps of a high magnification, if I'd given it even the slightest thought I would have realised that it was best to buy a lower magnification then barlow it up. The Eps that I bought were too high in magnification to benefit from a barlow, in effect with a bit of foresight I could have had 2 good eyepieces for every one I'd bought.
Not only does a good barlow double your Ep collection it also allows you to keep the eye relief of the initial eyepiece. In English, eye relief refers to the distance the eye must be from the lens to see the whole field of view. Generally as magnification increases in eyepieces the eye relief shrinks, forcing you to strain your eye closer and closer to the glass and ultimately makes for a more uncomfortable viewing experience. The beauty of the barlow is in the retention of the more comfortable eye relief, in short a more relaxed observer is a happier observer.
While busily extolling the virtues of the humble barlow I am aware that there is a large sub group within the astronomical world who are loathe to use them. They insist (and not completely without cause) that barlows degrade image quality, after all the more glass you place between you and the object you wish to observe the more room there is for all sorts of unwanted and unnatural phenomena including blurring, scattered light and generally degraded images (akin to smearing vaseline over the Ep lense). While there is a slight amount of these aberrations present in the TAL barlow it is so insignificant and minimal as to be almost worth no mention at all.
All but the most hawk eyed of observer will find the instant boost to magnification worth the slight bit of ghosting on the brightest objects.
In conclusion the TAL is a superb barlow and is a steal at only £35. There is a very (and I do mean very) small amount of distortion over using a lone eyepiece but this is more than made up for by the instant increase in magnification and being able to use the eye relief of a lower powered eyepiece in conjunction with a highly magnified view.
The TAL does have a rather cheap plastic look to it and it's light weight seems to allude to a cheap barlow in every sense of the word, however once you get it on your telescope and use it with a variety of eyepieces you'll soon be reminded of the old saying "looks can be deceptive".