I have to say, whilst Sellswords is by no means the worst adventure I've seen, it's not exactly a scintillating example of adventure design. I've read it rather than run it, although I intend to do the latter at some stage.
There are some problems (maps far too cramped, treasure placement a decidedly random affair and inconsistent with the encounter design principles of 4e) that are easily resolved. Punjar certainly makes for an interesting setting for a city, but too little detail is given in the adventure about the major NPCs and the world around the adventure itself. Some of the numbers given for non-combat tasks are absurdly high. Like I say, easily fixed but a nuisance.
There are a few more problems that are less easily resolved. The initial run up to the dungeon is extraordinarily brief. The encounters are oddly balanced, with few fights meeting the XP bill to make up a challenge for a group of even first level, apparently justified by the expectation that fights will often merge together. In addition, there are some elements such as poisons with jarringly powerful, lasting effects. The result is an encounter balance that seems to swing unpredictably between absurdly easy and unnecessarily lethal. The whole seems less skillfully planned than 4e makes possible.
I've also got a fair few problems with the formatting, although these are mostly just little complaints that have no impact on the adventure itself. The encounters often refer to one another, with some clearly intended as one encounter but inexplicably split over two areas, which is confusing and unhelpful. In some cases, one encounter will refer to elements of another area that don't exist or aren't listed in any great detail. Some of the stat blocks claim levels that they patently don't merit (to all appearances as an excuse for awarding a higher XP value, which could just be arbitrarily assigned - an approach that the adventure takes elsewhere - rather than fiddling with the given levels). Treasure values fluctuate between essentially worthless to remarkably valuable, with little consistency.
All this having been said, some of the ideas embodied within the adventure were very interesting. A cautious and skilled GM could turn this into an adventure to be remembered by their players. Some of the encounters, with a little creative re-imagining, could be excellent and Punjar itself is certainly a thoroughly usable environment. I would recommend a liberal hand be taken in modifying the adventure as it stands. Elements could certainly be incorporated from other sources. There was a Dungeon adventure a while ago for 3.5 which included a chase across barges, the latter half of which was somewhat disappointing (generic dungeon crawl after a thrilling first half). This could be attached to the end of that fairly easily, providing Sellswords with a more gripping introduction and the other adventure (which I forget the name of - the secret of something... there was a key involved...) with a better conclusion.
[Edit: I've found it. Mad God's Key, Dungeon 114]
My best luck with 1st level 4e adventures has been had with a conversion of the "Burning Plague" 3rd edition adventure available on the Wizards website. It's pretty easy to modify and re-map, and remains a passable - if generic - dungeon crawl.