If you already have your heart set on a particular LEGO set, by all means buy it! However if you are new to the LEGO hobby and just wondering what makes most sense to buy, I can share with you some criteria I personally use when I go LEGO shopping. (These can also be used if you are buying LEGO for someone else.)
WHAT IS YOUR NICHE?: If you have a somewhat limited budget, I would start with this question; what is it you like to build? LEGO Spaceships? Houses? LEGO Castles? Ships? Trains? Mosaics? Cars? Robots? Or how about life-size LEGO sculptures? Depending on the answer, I would look at the currently available LEGO sets and see what would give me the greatest amount of useful pieces in that theme.
For example, if you really like to build spaceship-type vehicles, I would recommend buying LEGO Star Wars Sets, even if you don’t really care about Star Wars per se. The Star Wars theme has a lot of pieces that can be used and reused again and again for LEGO spaceships of your own design. Suitable colors, angled plates, engine parts, streamlined windscreens, wheels, antennas, etc. – lots of pieces to work with!
Or, if you like to build castles, I would recommend picking up something from the LEGO Castle Collection. They give you wall elements, arched windows, roof pieces, and other parts that can be incorporated into your own castles.
And if you really like to build realistic city buildings, I highly recommend the limited edition LEGO sets that have been specifically targeted to adult fans of LEGO, like the #10197 LEGO Grand Emporium, or #10214 LEGO Tower Bridge. They are full of unique and useful pieces and colors for town-building. Also, they often incorporate advanced building techniques you can enjoy and learn from, not available in sets targeting younger fans.
Then there is also a special line of LEGO called LEGO Technic, full of interesting pieces for functional and realistic building of model cars, trucks, motorcycles, cranes, etc. - often with power function!
COLOR SELECTION: My other criteria for buying a particular LEGO set would be the colors available in the set. If I’m after gray pieces for my own building, I would look for sets that have a lot of those pieces. Even if it is in a theme I don’t really care about.
PRICE: Each LEGO set has a total piece count of the set posted on the outside of the box (usually under the set number). You can use this number to calculate if the set is a good value for your money. Just divide the price of the set with the number of pieces in the set. So, for example the Star Wars #10188 LEGO Death Star has a total piece-count of 3803. If the retail price is $400, then $400 divided by 3803 is 0.10 or 10 cents a piece.
Licensed sets like LEGO Star Wars and LEGO Harry Potter tend to go for higher prices; 10-13 cents /piece. Non-licensed sets hover between 7-10 cents a piece. So, if you can find them for less, you got a bargain! Many department stores that carry LEGO have regular sales (like around holidays or when they need to move old inventory). It is worth to wait for these sales to get the most parts for your buck!
SETS OR INDIVIDUAL PARTS?: In general it ends up being cheaper to buy LEGO sets (especially on sale), if most of the parts in that set are going to be useful for your own building style. However it is possible that you won’t find a set that really meets your needs or you only need very specific parts, not available from any one set.
This is the time to hit BrickLink and check out the availability there. You will most likely find everything you would ever need, but the prices may be higher, especially for rare parts. So you have to see what makes most sense.
LEGO itself has a PICK-A-BRICK section on their website, where you can find a decent selection of currently produced parts and buy LEGO by the piece. Also, at LEGO’s own retail stores there is a PICK-A-BRICK wall, where you can buy pieces by the cup for a flat fee. (They are called PUB-cups and come in two sizes). You can check it out here: Shop for specific LEGO bricks and elements using PICK-A-BRICK at the Official LEGO Shop!
You may also want to check out these posts for further ideas:
Hope this helps some in your own LEGO buying excurtions! And if you have any questions, comments or tips for other readers, share it in the comment section below!