Build it yourself is probably the best option, but this assumes that you are talking a desktop (or small form factor) not a laptop.
Dell is a matter of getting yourself a good deal (they sometimes offer really good value for money deals) but if you tack on upgrades to a particular base configuration be sure to check with component retail to see if you can't get a better deal by simply choosing a plain base configuration and installing upgrade components manually.
The plus point is specs and price, and an eye for using good quality kit. The downside is that the machine is not always put together quite as thoughtfully as one would like, presumably to drive price down or accommodate the need to do the shiny dance. For instance looking at a studio 15: laptop case and lid are not quite as sturdy and unyielding as you would like, plus on the one hand the screen is located at about the same height as the keyboard (as opposed to having the screen raised firmly above the keyboard) which means a less ergonomic viewing angle to the screen itself and on the other hand the construction of hinges means that the screen cannot tilt as far back as more conventional designs (which exacerbates the previous issue). But to illustrate the plus points using the same machine: keyboard gives good tactile feedback, laptop motherboard is definitely a notch above what you would expect from a laptop at the price and you get a Core i3 and DDR3 RAM with dedicated graphics card. Battery life is “average”. Mediocre by some standards, but quite good for “desktop replacement” type.
Last time I bought an HP (desktop) it did its job fairly well, considering. As far as specs go, HP has a habit of doing it almost but not quit right (specs of CPU/RAM/disk and connectivity all make solid sense but they might drop the ball on for instance graphics; again, presumably to get the price point down). The upside is that if you don't need to meddle with the innards of the case (much), then you get kit that is put together sensibly, runs decently, in all probability can withstand a little accident, and will last its time.
The big downside to HP will only be apparent to those who want to do (semi-frequent) upgrades. With their desktops my experience has been that they're cutting corners on things like PSU (not much leeway for power hungry upgrades), and motherboard (for instance using only 2 RAM slots instead of 4, few PCI expansion slots etc.), and wireless kit. Also their internal layout can be a pain to work with. (who thought it a smart idea to flip a hard-disk enclosure on its side and mount it vertically so the lower front half of the interior of the case is basically unusable??)
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